Xen is a free software virtual machine monitor for IA-32, x86-64, IA-64 and PowerPC architectures. It 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, NetBSD and Solaris 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. XenSource, Inc was acquired by Citrix Systems in Octobe