These are the packages that would be merged, in order:

Calculating dependencies ... done!
Dependency resolution took 1.13 s (backtrack: 0/20).

[ebuild  N     ] sys-fs/lvm2-2.03.39::gentoo  USE="readline udev -lvm -nvme -sanlock (-selinux) -static -static-libs (-systemd) -test -thin -valgrind -vdo -xfs" 2811 KiB
[ebuild  N     ] sys-boot/grub-themes-gentoo-1.0-r2::gentoo  5117 KiB
[ebuild  N     ] sys-boot/grub-2.16-r1:2/2.16-r1::gentoo  USE="branding device-mapper fonts nls themes truetype -doc -efiemu -libzfs -mount -protect -sdl -secureboot (-test) -verify-sig" GRUB_PLATFORMS="efi-64 pc xen -coreboot -efi-32 -emu -ieee1275 (-loongson) -multiboot -qemu (-qemu-mips) -uboot -xen-32 -xen-pvh" 14380 KiB
[ebuild  N     ] app-emulation/grub-xen-host-1.0-r2::gentoo  USE="-pvh" 0 KiB

Total: 4 packages (4 new), Size of downloads: 22307 KiB

The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by app-emulation/grub-xen-host-1.0-r2::gentoo
# required by =app-emulation/grub-xen-host-1.0-r2 (argument)
>=sys-boot/grub-2.16-r1 grub_platforms_xen