If your guests start talking to one another, then it still has to be permitted by the ultravisor or the network layer. Isolation is subjective and PEF/Ultraviso

If your guests start talking to one another, then it still has to be permitted by the ultravisor or the network layer. Isolation is subjective and PEF/Ultravisor permit you to separate the administrators of the hypervisor from the administrators of the guest workloads. This is something you don't really get elsewhere with the same security guarantees and/or open implementation.