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

Xen 4.0.1 is a maintenance release in the 4.0 series.