Adding encryption to Btrfs

Adding encryption to Btrfs

exactly. AES is a block cipher, and XTS is a cipher mode. XTS is one way to use AES, and doesn't rule out hardware-accellerated AES at all, afaik. Wikipedia has a good page of description about cipher modes.

Authenticated encryption modes would also be great, as they're tamper-evident -- any modification or damage to the ciphertext results in unreadable data (aka "⊥"), rather than returning nonsense cleartext.