The point about security is that it seems to present the same failure as kvm is for Linux kernel. If the hypervisor is in the ring 0 you have the risk of VM escape from one to another or the host itself.
How do you mitigate that risk?
How do you mitigate that risk?
VMM has no more capabilities than the VM itself, thus a VM escape would be, outside of academics, of no value.
Refer to pages 8 to 10 in the OP PDF.