I'm unconvinced it is. All of those virtual hardware drivers require their own code, often at kernel privilege. Hypervisor functionality at the processor level helps but is a huge attack surface in itself. X86 "rings" are under tested because it's hard to - many violations just kill your whole VM, but more importantly there's a ton of instructions that access "physical" memory and the state of the page mapper, even at the ring-0 level, matters a lot.
I do think VMs are likely more secure than containers in a cloud environment, because of course at that level you have both problems, but I don't believe that.the number of vulnerabilities found at the VM layer is at all reflective of their actual vulnerability.
It simply is. Even in a heavyweight VM with lots of hardware support, those hardware drivers are a tiny fraction of the user/kernel interface, no matter how you choose to count it (lines of code, number of foreign calls, number of exposed modules).
If you don't want to derive this axiomatically, fair enough: count vulnerabilities. The tally you're looking for is every Linux LPE versus every Linux KVM escape.
