Lighter how? A VM and a process look almost the same from the running software's point of view. A process is basically the same as it was in DOS - one big memory space, except some memory regions are off limits, and some regions contain kernel trampolines (assembly is still fair game). QEMU/KVM just hooks basic things like MMU/page lookups, and interrupts (using CPU extensions, iff available), so hardware interfaces (ex, PCI) can be simulated, but otherwise the VM runs as a normal process using the kernel's scheduler. Xen just re-implements some of the scheduler stuff on the theory that not everyone needs the full kernel. Unless CPUs start incorporating scheduling (preemption of processes/VMs) and memory-space isolation and kernel interfaces (like fork, and k/exec), I'm not entirely sure what an "OS-less VM" might look like. (It would be great if half the kernel - ex, the Xen part - could be BIOS/CoreBoot-level firmware though, as it'd be nice to just use kernel drivers and common scripts to boot, etc, rather than Grub-specific or iPXE-specific ones, etc...)