Nested Virtualization support on EC2 Graviton 3 *.metal instances

In terms of Graviton3 which utilizes the Architecture ARMv8.4-a [1] from the 8.3 version onward nested virtualization is supported. This is only the case for the Bare Metal instances, as you mentioned.

From a hardware and architecture perspective this is supported. It would come down to the software that you are using to perform this.

References: [1] https://developer.arm.com/documentation/102142/0100/Nested-virtualization [2] https://phoenixnap.com/kb/ubuntu-install-kvm [3] https://ubuntu.com/blog/kvm-hyphervisor