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

Xen 4.1.2 is a maintenance release in the 4.1 series and contains:

Stability improvements and bug fixes in

New XL toolstack
Debug support: kexec/kdump
Remus (High Availability)
Device passthrough to HVM guests
Interrupt handling
Support for Supervisor Mode Execution Protection (SMEP)

It also contains the Xen 4.1.1 fixes:

Security fixes including CVE-2011-1583 and CVE-2011-1898
Enhancements to guest introspection (VM single stepping support for very fine-grained access control)
Many stability improvements, such as:

PV-on-HVM stability fixes (fixing some IRQ issues)
XSAVE cpu feature support for PV guests (allows safe use of latest multimedia instructions)
RAS fixes for high availability
fixes for offlining bad pages
changes to libxc, mainly of benefit to libvirt

Compatibility fixes for newer Linux guests, newer compilers, some old guest savefiles, newer Python, grub2, some hardware/BIOS bugs.