My vague understanding was that KVM-accelerated virtualization still let you use Virtio RNG, although I guess I don't know what the relative performance looks like at that point.
Virtio-rng is good for getting quality seed material into the guest, but you don’t want to use it for bulk generation. Some hypervisor environments impose arbitrary and low throttles on guest virtio-rng use (think kB/s).
Even with paravirtualization as an option I feel like many users of VMs would opt for the advantages of full virtualization.