Xen 4.0 was just released
a few days back with a variety of features from graphics card pass-through support to online resizing of guest disks, but features for Xen 4.1 are already brewing. Xen 4.1 will be the next major release for this once-popular virtualization platform and its feature list is quickly growing.
Among the proposed features so far is support for pass-through to secondary graphics cards, pulling the latest upstream QEMU code into qemu-dm that alone will provide a variety of improvements, porting the SPICE VDI project to Xen, XenFS coming to fruition, block device throttling/scheduling, improve QCOW2 support, and nesting support of VMs. As part of the secondary GPU pass-through support would be support for loading a video card BIOS from a file and pBAR->vBAR mapping.
Not all of these features may end up making the cut for Xen 4.1, but for some features (like the DomU nesting within a Dom0) there is already code in the works. There may also be other features that come about for Xen 4.1 before it's officially released at some point in the future.
The list of potential Xen 4.1 features can be found on the Xen Wiki road-map