In PV (paravirtualized) mode, the guest OS does not attempt to access hardware directly, but instead makes hypercalls to the hypervisor; PV guests must be specifically coded for Xen. See PV. PVH is substantially more efficient than PV because it uses hardware assisted virtualization. There have been two PVH modes: original PVH and PVHv2. Original PVH was based on PV mode and is no longer relevant