The Xen 4.0 release contains a number of important new features and updates including:

Blktap2 – High performance VHD implementation supporting snaphots and clonces including live snapshots
Netchannel2 – Support for new Smart NICs with multi-queue and SR-IOV functionality
Fault Tolerance – Live transactional synchronization of VM state between physical servers
Libxenlight – New library providing higher-level control of Xen between various toolstacks
PV-USB and VGA Pass-through
Xen 4.0.x Release Notes
Xen 4.0 Datasheet