The Xen 3.4 release contains a number of important new features and updates compared to the 3.3 release including:

• Device passthrough improvements, with particular emphasis on support for client devices (further support is available as part of the XCI project).
• RAS features: cpu and memory offlining
• Power management: improved frequency/voltage controls and deep-sleep
• support. Scheduler and timers optimised for peak power savings.
• Support for the Viridian (Hyper-V) enlightenment interface
• Many other changes in both x86 and IA64 ports
• Xen 3.4 Datasheet

Xen 3.4.2 is a maintenance release in the 3.4 series.