There are some providers that offer KVM nested virtualization, I think Google Cloud, Digital Ocean ... any others?

True! GCP does. I haven't tested it yet. I didn't know D.O does. If anyone knows others, I'm interested too!

We (NanoVMs) can run (both unikernel and normal linux) virtualized workloads on plain old ec2 instances (eg: t2.small).

Interesting, thanks for sharing!