Qubes uses the type-1 Xen hypervisor that runs at a similar privilege to the kernel of other OSes. KVM is a type-1 hypervisor implemented as a Linux kernel module.
What tells them apart them? When would you use one vs the other?
Perhaps Xen for having all machines, including the one that controls the hypervisor, being virtualized, as opposed to KVM/QEMU running on the control bare-metal with VMs on top?
I remember when google search had one text-based ad. Bearable.