Xen is an open source virtual machine

Xen

Xen is an open source virtual machine monitor for x86-compatible computers. XenSource Inc. and Virtual Iron Software Inc. promoted Xen as the primary open source competitor to commercial virtualization products such as VMWare.The hypervisor acts like a traffic cop, directing hardware access and coordinating requests from the guest operating systems.Xen makes it possible for multiple guest operating systems to run on a single computer by using a software layer called a hypervisor to mediate access to the real hardware.

Xen, which was released under the GNU General Public License, was originally a research project at the University of Cambridge. XenSource, Inc., a company that supported the development of the open source project and enterprise applications of the software, was acquired by Citrix Systems in October 2007. Sun Microsystems provides support for Xen virtualization on Solaris 10, its version of the Unix operating system. Other mainstream Linux distributions, including Debian and SuSE, have the necessary kernel extensions available to serve as the base OS for Xen.