Specific to Geth the database used is LevelDB. But what is the database used for parity?
I'm one of parity devs. We use Rocksdb. It's a key-value database developed by Facebook. We chose it mostly because of it's performance. In our benchmarks it was faster than LevelDB
From their webpage:
RocksDB is optimized for fast, low latency storage such as flash drives and high-speed disk drives. RocksDB exploits the full potential of high read/write rates offered by flash or RAM.