Not quite, UML is the linux kernel ported to be an executable that runs under Linux. It's design kind of requires the system calls from Linux to be able to run

Not quite, UML is the linux kernel ported to be an executable that runs under Linux. It's design kind of requires the system calls from Linux to be able to run properly. Making a translation layer for that isn't impossible but it would add more overhead to running things that way. Running a VM in a hypervisor, you're instead creating a whole virtual computer and its IO interfaces instead of at the system call level. Because there's hardware support for doing that virtualization (basically the CPU can help isolate the new process so that it can't see real hardware and the hypervisor gets notified/pulled in when needed to emulate the IO) it can end up nearly native speed if setup correctly.