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 4.0.3 is a maintenance release in the 4.0 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 4.0.2 and older 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.