Xen 4.0 is supposed to be able to use Citrix’s WHQL certified Windows paravirtualization drivers. Their advantage over the GPLPV drivers is that they are code-signed, meaning they run on 64-bit Windows without disabling some of Windows’ security features.

UPDATE 2011-10-17: Signed GPLPV drivers are now available. I have not yet tested them, but I assume the fix below is no longer necessary.

While the Citrix drivers included in XenServer 5.5 work (after making a single registry tweak), the more recent ones included in e.g. Xen Cloud Platform 1.0 do not work right away:

If you install the XCP drivers, make that registry tweak and reboot the DomU, you’ll notice messages like XENUTIL: WARNING: CloseFrontend: timed out in XenbusWaitForBackendStateChange: /local/domain/0/backend/console/[id]/0 in state INITIALISING; retry. in your /var/log/xen/qemu-dm-*.log and Windows just gets stuck during boot and keeps spinning forever. To get it back to work, you’ll need to
xenstore-rm /local/domain/0/backend/console/[id]
xenstore-rm /local/domain/0/backend/vfb/[id]
after starting the VM (thanks to Keith Coleman‘s mailing list post!).

To automatically run these commands upon DomU start, create a script named /usr/lib/xen/bin/qemu-dm-citrixpv with the following contents
#!/bin/sh

exec /usr/lib/xen/bin/qemu-dm $*
and chmod +x it.

Then, edit your DomU config file and modify the device_model line and point it to your new script:
device_model = '/usr/lib/xen/bin/qemu-dm-citrixpv'

Now your Windows Server 2008 R2 x64 HVM-DomU is all set!