A GPL'd Xen paravirtual driver for Windows is under active development.

The leading developers are James Harperfor from Bendigo IT and Andy Grover from Oracle.

Oracle has other developers involved in the driver development too. Oracle VM is planing to support it once it is ready for production use.

GPL'd Windows paravirtual driver Mercurial (hg) source repository is on:

http://xenbits.xensource.com/ext/win-pvdrivers.hg

Driver developers sometimes publish binary release. Eg. this latest one (on 2008-03-06) is:

http://www.meadowcourt.org/WindowsXenPV-0.8.4.zip

Driver development is discussed in mailing list xen-devel:

http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel/

You can seach the mailing list archive at:

http://lists.xensource.com/archives/html/xen-devel/