New kernel for dom0 - instead of ancient 2.6.18, we get a new subsystem built on PVOps interface of new Linux kernels. Currently 2.6.31.13 is supported, but porting it to a new version is now trivial compared to the old approach. Finally Xen gets a decent support for new hardware and running a modern distribution in dom0 is way easier.
Blktap2 - new disk I/O system. This almost deserves a separate post. Virtual block devices (hard disks) can be backed by files, partitions, LVM volumes or even network devices. VHD, the native format of Microsoft virtualization systems and a de-facto industry standard (VirtualBox and VMware can read it too) is now supported. Advanced disk formats such as copy-on-write (run multiple VMs from the same image initially and only copy the blocks that changed), sparse images, on-the-fly encryption and compression are easier and faster then before. Disk images can be accessed from outside the VM. But the real killer feature is live snapshot.
Copy-on-write RAM. VMs can now share memory. The scalability is also improved: by default Xen supports 1 TB RAM and 128 CPUs, limits can be increased.
Improved PCI passthrough - PV guests still use old mechanism by default cause it doesn't require hardware support. If your CPU and chipset combination support IOMMU, you have to enable the new way manually, see http://wiki.xensource.com/xenwiki/XenPCIpassthrough
Many other improvements. Updated Pygrub, certified PV drivers for Windows guests, disk resize without reboot, libxl - new userspace Xen management API, improved networking, physical CPU and RAM hotplug, support for Smart NICs with multi-queue and SR-IOV (one physical Ethernet card appearing as multiple PCI devices)
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