So, given I'd never heard of xvisor I think we can say that no one has been thinking about supporting it.

Nova has a pluggable layer for adding hypervisors, and its certainly possible to do that thing. That said, Nova is pretty hesitant these days to add new hypervisor drivers, as its a lot of maintenance work for the team, especially if the original authors of a driver wander off.

A first port of call is probably to work out if you could get support added to libvirt, and then what exposing that in the libvirt driver for Nova would look like.