I know Nested Virtualisation is a thing on both KVM and hyper-v, what is different about what you could do on "big iron"

In the early days of virtualization on PCs (things like OS/2's dos box) the VM was 100% a weird special case VM that wasn't even running the same mode (virtual 8086 vs 286 / 386 mode), and that second-class functionality continued through the earlier iterations of "modern" systems (vmware / kvm / xen).

"PC" virtualization's getting closer to big iron virtualization, but likely will never quite get there.

Also -- I was running virtual machines on a 5150 PC when it was a big fast machine -- the UCSD P System ran a p-code virtual machine to run p-code binaries which would run equally well on an apple 2. In theory.

A VM nest in "big iron" isn't a special case. It's a context push with comparatively exhaustively defined costs, side effects, and implications.

IMO, it’s only a special case for commercial support reasons. Almost every engineer, QE, consultant, solution architect I know runs or has run nested virtualization for one reason or another.

And licensing - DB2 and Oracle.