In its default config XenServer 6.5 will not permit you to use your host server’s primary GPU in a VM as a passthrough device. That’s reasonable in most cases, as reusing this GPU leaves the dom0 without output. However, passing through the device works nonetheless in many cases – your mileage may vary.
To convince XenServer to let you freely assign the GPU to VMs add this line to the file /etc/udev/rules.d/05-udev-early.rules:
Reboot the server, that’s it.