Explain the difference between Extreme programming and Scrum?

Answer 1

Scrum is a software development methodology, XP is a programming practice. Both are "agile" techniques and are often used together. Please find the following high level differences -

1. Scrum typically work in iterations (called sprints) that are from two weeks to one month long where as extreme programming typically work in iterations that are one or two weeks long.
2. Scrum do not allow changes into their sprints. Once the sprint planning meeting is completed and a commitment made to delivering a set of product backlog items, that set of items remains unchanged through the end of the sprint. XP teams are much more amenable to change within their iterations. As long as the team hasn't started work on a particular feature, a new feature of equivalent size can be swapped into the XP team’s iteration in exchange for the unstarted feature.
3. Extreme Programming teams work in a strict priority order. Features to be developed are prioritized by the customer or Scrum’s Product Owner and the team is required to work on them in that order. Whereas the Scrum product owner prioritizes the product backlog but the team determines the sequence in which they will develop the backlog items.
4. Scrum doesn’t prescribe any engineering practices; XP does.

Answer 2

Adding few more points to Salil's answer:

1. Requirements/Changes are well documented in Scrum in terms of stories created by product owner but in XP it is not. This can cause conflict at later stages (UAT).
2. Scrum is an agile project management methodology but XP is an agile software development methodology.

So in short, when mixing Scrum and XP, which is by far the most common mix, you use all of Scrum's management artifacts, e.g. Sprints, daily Scrums, retrospectives, burn down charts, and so on, and add XP's TDD, refactoring, pair-programming and JIT design via User Stories.