This is never going to happen though. Not because Windows PV is not possible. Windows was ported to Xen PV in the distant past because Cambridge University had a Windows source license.

The reason it's not going to happen is because with the latest chips and 64 bit, hardware full virtualization is faster than paravirt. Paravirt on x86-64 can't use the i386 segmentation trick, because x86-64 (long mode) doesn't have full support for segment registers. So it has to use some other trick which is slower.