Xen 4.9 multiboot2 support increased

At least one UEFI change in this release:

Boot Xen on EFI platforms using GRUB2 (x86):
From Xen Project 4.9 and GRUB2 2.02 onwards, the Xen Project Hypervisor can be booted using the multiboot2 protocol on legacy BIOS and EFI x86 platforms. Partial support for the multiboot2 protocol was also introduced into network boot firmware (iPXE). This makes the Xen Project boot process much more flexible. Boot configurations can be changed directly from within a bootloader (without having to use text editors) and boot configurations are more portable across different platforms.

https://blog.xenproject.org/2017/06/28/whats-new-in-the-xen-project-hypervisor-4-9/