It's really about as simple, and as bare bones as you can make a traditional KV storage while still preserving MVCC, transactions, and maintaining ACID.
Furthermore it's design is prefectly suited to exploit Optane/FRAM non-volatile RAM devices that appear to becoming.
LMDB does mmap while LevelDB does plain file I/O.
Every `malloc` under XEN (that can't be satisfied with unused memory already mapped into that processes userspace) requires a hyper call.
This VMM approach was _largely_ depreciated about 6 years ago when Intel created page table bits that let the bare metal OS give the VM a _range_ of memory it can virtually re-map, but AWS hasn't adopted this.