Until recently, Windows users who wanted to get into the action were out of luck because Xen ran only on Linux and NetBSD servers. All that changed with the formation of XenSource, a commercial open source company launched by Xen's creators that has developed an extension of Xen 3.0 called XenEnterprise, which runs x86-based operating systems -- including Windows -- without requiring operating system modifications. Linux and Solaris are also supported, although the free Xen 3.0 supports Linux as well.

There is one catch that could be a showstopper for some: The x86 servers need to use Intel CPUs that include the VT-x hardware virtualization technology -- including some Xeon MPs and forthcoming Itaniums -- or AMD CPUs that have the SVM (Secure Virtual Machine) technology.

All this hardware support eliminates the need for performance-sapping chip-set emulation in software to run the virtualized "guest" operating systems. "XenEnterprise is like the Windows HAL [Hardware Abstraction Layer]. So we can do bare-metal I/O performance," says XenSource CTO Simon Crosby. He claims the XenSource virtualization is a third faster than EMC's and Microsoft's virtualization products.

Microsoft's decision to license to XenSource its Virtual Hard Disk file format, used to capture virtual server images and pass them around physical boxes, gives Windows-based IT shops further confidence in their ability to rely on Xen virtualization. It also pits XenSource head-to-head against VMware, which has its own virtual machine disk format.

Xen's approach to virtualization has fueled wide interest on the part of both hardware vendors and IT users. With the commercial version's Windows support, XenSource could make the resource efficiencies of server virtualization available without performance penalty. Even better, Xen's open source roots might mean virtualization doesn't have to penalize IT budgets, either.