Would be very surprised if it's not backed by AWS HSM. About 2 years ago I believe AWS KMS-->HSM was leveraging SafeNet's Luna HSM.
The SafeNet product is now called "CloudHSM Classic". The current CloudHSM is documented to use the Cavium NITROX HSM, which in the PCI HSM format is partitionable into "virtual HSMs". This looks pretty much designed for cloud HSM providers.