[libvirt] libvirt: how to set PV-on-HVM drivers??

Hi friend. I have a problem about using libvirt now. There is a feather about HVM driver ---PV_on_HVM_drivers, in xm configure, using flag xen_platform_pci can control the feather, but in libvirt I can not find the flags to use pv-on-HVM drivers. So I want to find the answer if the libvirt can control the modules using xml file? thanks a lot. best regards. zhong.chunshan http://wiki.xen.org/wiki/Xen_Linux_PV_on_HVM_drivers