blktap3 is an improved version of blktap2. The main difference between blktap2 and blktap3 is that the tapdisk process running in dom0 is directly connected to blkfront, which means that blkback is no longer on the datapath longer in this configuration. The advantage of this approach is improved performance since requests do not have to pass through the dom0 kernel.
blktap3 in under heavy development; it will soon be sent as a patch in xen-devel. blktap3 seems to work on PV VMs without pygrub.
Need to implement support for HVM VMs, PV pygrub VMs.