Create the most possible value for the customer while mitigating the risk of project failure
Continually Improve the development processes
Flatten the Cost of Change curve
XP includes 12 basic practices which interlock and reinforce each other to support the values
Software often lives far longer than any of the original developers imagined. Many of the XP practices aim at reducing the cost of making changes to the system; hopefully keeping it constant; enabling the software and technology to gracefully evolve with changing business requirements.
XP software is essentially in 'maintenance' mode after the first three weeks of development.