With the upcoming Linux 3.17, it's possible to boot using (U)EFI under Xen Dom0. Daniel Kiper who worked on the Xen EFI patches explained, "Standard EFI Linux Kernel infrastructure cannot be used because it requires direct access to EFI data and code. However, in dom0 case it is not possible because above mentioned EFI stuff is fully owned and controlled by Xen hypervisor. In this case all calls from dom0 to EFI must be requested via special hypercall which in turn executes relevant EFI code in behalf of dom0."
Other EFI changes queued up for the Linux 3.17 merge window are outlined in this EFI pull request but the Xen EFI Dom0 support seems to be the main user-facing feature this cycle. After being introduced in Linux 3.16, there's also continued work on 64-bit ARM EFI support for 3.17.