It should be noted, since no one else seems to have brought up the point, but Xen was originally partly funded by Microsoft. The original history of Xen had it running on both Red Hat Linux and Windows XP. IIRC They used the Shared Source program available to educators to access the source and at the time XP was enabled as a Xen hypervisor client, I don't believe it could act as the hypervisor at the time though.

I quote from the xen development website: A port of Windows XP was developed for an earlier version of Xen, but is not available for release due to licence restrictions.