XEN was developed by a bunch of British 'cheps and chepettes' at Cambridge Uni'. They are still working with XEN, but have now launched XENEnterprise.
XEN is a Hypervisor. It is implemented from within a Linux operating system. You install SuSE or Red Hat (must be the Enterprise versions appvshnotify safe or not and no earlier than the latest releases), and after the installation you can choose to recompile the kernel to include the XEN extensions/code.
XEN has a very small footprint. It is under 60,000 lines of code. It's a lean, mean virtual machine, and it is, probably, the best of all the virtual hosts currently available. I say this because it offers ParaVirtualisation, simple installation, near metal performance, and equals the low overhead of XENEnterprise. In fact, some figures show that it is more efficient than XENEnterprise.