Don't use a tap interface, but Snabb does have a fast virtio driver for kvm, that maybe you could adapt (or use directly if you dont mind the userspace parts be

Don't use a tap interface, but Snabb does have a fast virtio driver for kvm, that maybe you could adapt (or use directly if you dont mind the userspace parts being in another vm). Or just do everything in userspace. Reinjecting into the kernel is not strictly necessary, although obviously you may be constrained by existing code.