Webopedia: All About Hypervisors

What is Virtualization?
In plain and basic words, virtualization is used to improve IT throughput and costs by using physical resources as a pool from which virtual resources can be allocated. If that is still a bit too much to swallow, consider then, the simple scenario of partitioning a computer hard drive. A single physical hard drive is partitioned and it effectively creates two separate hard drives in terms of how it is seen. While you still have only a single physical hard drive in your system, this act of virtualization allows the computer system, devices and human users to work with that single hard drive as if it were two physical hard drives.

Virtualization, of course, goes far beyond the simple act of partitioning a hard drive however. This year storage virtualization has been a hot topic. Storage virtualization is the amalgamation of multiple network storage devices into what appears to be a single storage unit, which is somewhat of the opposite effect of partitioning a hard drive. There is also server virtualization, which is the partitioning a physical server into smaller virtual servers. Network virtualization uses network resources through a logical segmentation of a single physical network. And the list goes on to cover operating system virtualization and even application virtualization.

System Virtualization: Hardware & Hypervisors
System virtualization is a technology often used to consolidate systems, workloads and operating environments by using a single physical system to create multiple virtual systems. One type of system virtualization is done through hardware partitioning, which divides a single physical server into partitions — where each partition is able to run an operating system.

Hypervisors (also called Virtual Machine Monitor (VMM) or virtualization manager), is another technology at heart of system virtualization. A hypervisor provides the underpinnings for virtualization management, which includes policy-based automation, virtual hard disk, life cycle management, live migration and real-time resource allocation. It's the software program or part of the code in firmware that manages either multiple operating systems or multiple instances of the same operating system on a single computer system. The hypervisor's job is to manage the system's processor, memory and other resources to allocate what each operating system requires. Hypervisors provide the means to logically divide a single, physical server or blade, allowing multiple operating systems to run securely on the same CPU and increase the CPU utilization.

Where hardware partitioning allows for hardware consolidation, hypervisors allow for flexibility in how the virtual resources are defined and managed, making it a more-often used system consolidation solution.

Types of Hypervisors
IBM systems breaks hypervisors down into two different types. Type 1 hypervisors are those that run directly on the system hardware and offers a higher level of virtualization efficiency and security.

Type 2 hypervisors are those that run on a host operating system that provides virtualization services, such as I/O device support and memory management. Type 2 hypervisors are used mainly on client systems where efficiency is less critical, and are also commonly used for systems where support for a broad range of I/O devices is needed and can be provided by the host operating system.

Figure shows one physical system with a type 1 hypervisor running directly on the system hardware, and three virtual systems using virtual resources provided by the hypervisor. [Image Source: IBM Systems Virtualization Paper (PDF)]

Figure shows one physical system with a type 2 hypervisor running on a host operating system and three virtual systems using the virtual resources provided by the hypervisor.
[Image Source: IBM Systems Virtualization Paper (PDF)]

For more information on types of hypervisors, we recommend reading this IBM Whitepaper resource, "IBM Systems Virtualization".

Current Players in the Hypervisor Market

VMware
VMware's virtual machine (VM) approach creates a uniform hardware image — implemented in software— on which operating systems and applications run. On top of this platform, VMware's VirtualCenter provides management and provisioning of virtual machines, continuous workload consolidation across physical servers and VMotion technology for virtual machine mobility. VMware offers a wide range of products for data center infrastructure, workstations, enterprise desktops, virtualization accelerators, as well as several free virtualization products.

VMware Web site

Xen
Xen is a virtual machine monitor (VMM) for x86-compatible computers. Xen is built to securely execute multiple virtual machines, each running its own operating system, on a single physical system with close-to-native performance. Xen is open source, and is released under terms of the GNU General Public License. Xen is a Type 1 hypervisor that runs directly on the system hardware. Xen originated as a research project at the University of Cambridge.

Xen Web site

Microsoft Virtual Server
Windows Server virtualization, as a part of Microsoft's "Longhorn" server, takes a big step forward in bringing some of the advanced capabilities of virtualization to bear and providing customers with a scalable, secure and highly available virtualization platform. Microsoft Virtual Server 2005 R2 is cost-effective server virtualization technology engineered for the Windows Server System platform

Microsoft Virtual Server Web site

Intel & AMD Offer Virtualization
Industry heavyweights like Intel and AMD have also given virtualization a huge credibility boost. For example, the two chip vendors are building virtualization capabilities into their chip architectures — Intel with its Intel Virtualization Technology (VT) and AMD with its AMD-Virtualization (AMD-V), on Xeon and Opteron processors, respectively. From an application standpoint, the hardware will enable applications that have previously been hard to virtualize (e.g., I/O-intensive apps like database applications for which the overhead has been too high) to be virtualized much more successfully. As Intel's and AMD's technologies are introduced, it will offer a choice of which virtualization path to explore Windows Server that of VMware, the market leader; open source software from XenSource; or an integrated Microsoft solution.

For more information on Intel Virtualization Technology (VT) and AMD-Virtualization (AMD-V) we recommend reading this ServerWatch Tutorial: 'Server Virtualization Goes Prime Time'.

Vangie 'Aurora' Beal - Writer, www.Webopedia.com
Last updated: November 03, 2006

ServerWatch: Server Virtualization Goes Prime Time
Virtualization technology is enjoying a period of explosive growth at the moment, and increasing numbers of enterprises are becoming virtualization converts. Research firm IDC estimates about 750,000 virtual servers were in operation in 2004, and it expects this to rise to more than 5 million by 2009 . a compound annual growth rate of almost 50 percent.

ServerWatch: Virtually Speaking: The Not-So-Humble Hypervisor
The pervasiveness of virtualization will not be limited to servers. Gartner analyst Brian Gammage noted in a statement, "Virtualization support is now being embedded into PCs. By the end of 2007, most of the PCs sold will come with this capability. The launch of hardware virtualization support marked the start of a period that will inevitably lead to the establishment of a critical new PC standard: the hypervisor."

VMWare Virtualization Overview WhitePaper (PDF)
Today, virtualization can apply to a range of system layers, including hardware-level virtualization, operating system level virtualization, and high-level language virtual machines. Hardware-level virtualization was pioneered on IBM mainframes in the 1970s, and then more recently Unix/RISC system vendors began with hardware-based partitioning capabilities before moving on to software-based partitioning.

Windows Server Virtualization . An Overview (Word Document)
Microsoft is delivering application development tools, server applications, operating systems, and management solutions that provide immediate improvements to address the complexity in customers. IT environment. As a part of the virtualization solutions, customers will see improvements in the current product offering for Virtual Server 2005 R2; new advanced products such as System Center Virtual Machine Manager that will address key management challenges and Windows Server virtualization as a part of Windows Server .Longhorn., that will provide an improved virtualization platform with increased scalability, performance and reliability.

Wikipedia: Comparison of virtual machines
The table below compares basic information about virtual machine packages, including: creator, guest systems supported, license, etc. Note that these are all Virtual Machines in the 'hypervisor' or 'hardware emulator' sense.