Right, but what x86 KVM setup is there in practice that doesnt present a mmu to its guest and hence keep its own track (shadow) of memory mappings?

It depends. If you have a plain old VM with no GPU and networked storage it's easy to migrate. If you're using low latency HW on the host it gets a lot trickier. Kuberenetes or some other fleet management helps there, you spin up new nodes and drain the old ones as capacity allows. If you have your PodDisruptionBudgets set right you won't notice much.

You can also use ksplice to live patch the vulnerable kernels as a first aid. I'm not an expert but I think there are some limits to what you can accomplish with it. Also guessing that the embargo period allows the big names to roll the patches out gradually enough that you won't see any panic shutdowns.

Yeah most cloud providers have live migration and a reasonably-well-tested playbook for updating the node kernel + hypervisor.