The Mark II model updated the regular RJ45 port to an Ethercon port. The rest is basically the same as the original Mark I.
The XL model added more storage memory for presets and cabs, uses flash memory for all storage, updated the value wheel to an optical encoder for longer life, added a second expression pedal jack, added dedicated MIDI THRU port, added FAS-LINK port, added emergency recovery firmware to recover from failed firmware updates (Mk I and II require a physical EPROM chip replacement if this happens). It supports more a few more block copies and a few more blocks have X/Y. It also has a lower noise floor due to an improved input circuit.
The XL+ model uses a different LCD screen and controller chips (old one was end of life). Main board layout was updated to accommodate the new LCD controller chips and this lead to an even lower noise floor. Everything else is the same as the XL.
The CPU's, RAM, and processing power are identical between them all.