http://www.xensource.com/
http://en.wikipedia.org/wiki/Xen

Xen is a free source virtual machine monitor for IA-32, x86-64, IA-64 and PowerPC architectures. It is software that runs on a host operating system and allows several guest operating systems to be run on top of the host on the same computer hardware at the same time. Modified versions of Linux and NetBSD can be used as hosts. Several modified Unix-like operating systems may be employed as guest systems; on certain hardware, as of Xen version 3.0, unmodified versions of Microsoft Windows and other proprietary operating systems can also be used as guests.

Xen originated as a research project at the University of Cambridge, led by Ian Pratt, senior lecturer at Cambridge and founder of XenSource, Inc. This company now supports the development of the open source project and also sells enterprise versions of the software. The first public release of Xen was made available in 2003.

It claims to lead the industry in performance, security and cross platform support.

Xen is set for ubiquitous adoption and to a de facto industry standard open source hypervisor.

The Xen project relies on commitments from a wide range of vendors including Intel, AMD, Cisco, Dell, Egenera, HP, IBM, Mellanox, NetApp, Novell, Red Hat, Sun, Veritas, Voltaire, and of course, XenSource.

http://community.citrix.com/download/attachments/85753928/DDM.jpg

Xen takes advantage of hardware OsVirtualization support from Intel® VT-x enabled processors (and AMD® Pacifica enabled processors in the future) to enable virtualized guests to run natively on the hardware while still achieving very high performance I/O. With alternative approaches, the hypervisor must binary patch running guests to prevent them from interacting with the hardware, resulting in high performance overhead and stability and security risks. Moreover, this approach results in significant I/O performance impact.

ParaVirtualization requires a tiny HyperVisor code base (Xen is under 50 KLOC) that results in extremely low performance overhead, typically in the range of 0.1% to 3.5% for industry standard performance benchmarks. It also leverages all of the native Linux device drivers and therefore supports an extremely diverse set of drivers. Xen’s paravirtualized drivers run outside the core hypervisor, where they implement policy for resource sharing between multiple guests, providing fine-grained partitioning of I/O between multiple virtual servers.