The Xen 4.1 release contains a number of important new features and updates including:

A re-architected XL toolstack that is functionally nearly equivalent to XM/XEND
Prototype credit2 scheduler designed for latency-sensitive workloads and very large systems
CPU Pools for advanced partitioning
Support for large systems (>255 processors and 1GB/2MB super page support)
Support for x86 Advanced Vector eXtension (AVX)
New Memory Access API enabling integration of 3rd party security solutions into Xen virtualized environments
Even better stability through our new automated regression tests
Xen 4.1 Release Notes
Xen 4.1 Datasheet