Xen uses QEMU just to emulate virtual hardware devices for HVMs, not for the actual virtualization. "Normal" Qemu is actually Qemu/KVM, which is not supported on Xen as far as I know. The next best thing is to create an HVM, see https://www.qubes-os.org/doc/standalone-and-hvm/#installing-an-os-in-an-hvm
qemu-img-xen is used for formatting image files or block devices for VMs. qemu-nbd-xen is for network block devices, though I'm not sure if/how they're used in Qubes.
...Xen uses QEMU just to emulate virtual hardware devices for HVMs, not for the actual virtualization. "Normal" Qemu is actually Qemu/KVM, which is not supported on Xen as far as I know. The next best thing is to create an HVM, see https://www.qubes-os.org/doc/standalone-and-hvm/#installing-an-os-in-an-hvm
qemu-img-xen is used for formatting image files or block devices for VMs. qemu-nbd-xen is for network block devices, though I'm not sure if/how they're used in Qubes.