What are the security risks of running QEMU/KVM as root?

Context: I own a machine; I trust root and all the accounts. I virtualize untrusted guests using KVM, and don’t want them to escape.

When /dev/kvm has the right permissions, non-root users can run KVM guests. Does this bring any security advantages over running guests as root? In case of a qemu or KVM vulnerability, won’t malicious guests gain kernel privilege no matter what user is running qemu?

Going further, assuming accounts of the host can’t be trusted, is it possible to gain root privileges using /dev/kvm?