How much training does an A320 pilot need to fly an A321? From flight simulation and from pictures, it appears that the cockpit of the  A318, A319, A320, and A321 have few if any differences.


Could an A320 pilot fly an A321 without further training?
If not, how much training would he need? Maybe he has to practice landing, because of the different length of the aircraft.
What big differences in the cockpit do those planes have?
From Flight Training International:


  Airbus A320 Type Rating  
  
  
  The Airbus A320 family of jet airliners consists of five aircraft:       the A318, A319, A320, A321 jet airliners and ACJ business jet.  
  Only one type rating is required to fly these Airbus aircraft, as       they have similar flight decks.  
  To earn a type rating for these Airbus aircraft requires the    completion of a technical course that lasts anywhere from 8 to 12 days depending on your pace.
  


So the answer from a regulatory point would be no additional training.  Some airlines might have policies that require a minimal check, probably in a sim.