I was forunate enough to see Scale in action from friend and saw his lab of multiple nodes connected.
Scale does use KVM, and virt IO that is not secret, everyone can have this tech right now, but hypervisor alone is not enough, reminds me abit of mobile processors and cpus, you can have the best cpu but without decent gpu + uncore + modem, it will be crap, and same can be said for scale they wrap KVM with amazing tech to make it hyper-converged in such easy manner. Those multiple nodes pool resources and you have very easy web GUI to manage VMs and easily move them and clone them and migrate them to other nodes.
So KVM or ESXi alone does not really cut it in this time and age. And scale magic is mostly in pooling storage in an effortless manner between nodes, and giving you all this in an easy web UI.