Finally!

From the official page:

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

SourceMage has already updated packages for it.

There’s a repository and src RPMs available for Fedora here. Specifically the Xen 4.0 src RPM. I’ll try to remember to fire up a build on my xen DomU that I use to build RPMs for me :)