There are benefits of booting ISO which only consist of qemu-kvm and other small utility tools compared to directly boot into disk image file, including:
1. Pause VM execution at runtime (Linux often has trouble in hibernating)
2. Minimal overhead on booting OS which lacks driver support (i.e, Minix, Windows XP on UEFI, Windows 2000)
3. Easy troubleshooting, you can still boot on the hypervisor ISO and ask help (assuming there are working browser or IRC client in the ISO)
4. No dual boot confusion
But it's so hard to find usable & almost baremetal hypervisor ISO, Xen requires dom0 OS which adds overhead, initial devuan debootstrap image result in >200 MB image file, and I still have trouble in using qemu-kvm on Tiny Core Linux
