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 3.4.4 is a maintenance release in the 3.4 series. It contains a number of fixes compared to 3.4.3:

Security enhancements including CVE-2011-1583
Support for AMD Family 15h (Bulldozer) CPUs
Performances and stability improvements
Various bux fixes