Joda-Time: what’s the difference between Period, Interval and Duration?

In Joda-Time 2, what is the difference between the three kinds of time spans:

• Period
• Interval

• Duration

  1. Why do we need three classes?

  2. Which one performs better?

  3. Why is dividing a Period or Duration or Interval instance not implemented? E.g. p = p.divideBy(2);