<p>What type of client is likely to support XP (Extreme Programming) practices?</p>

If your team achieves great results with a proven track-record, then companies desiring a successful result.  If the converse is true, only companies who are wandering blindly will be interested.
There is the
odd
case where the client will want a certain practices followed. Like a experienced dev manager outsourcing a project to an external firm, or potentially a client who has heard that XP is good in passing but has no real knowledge or experience with it. In the former the experienced consumer will know what he wants and if you do not provide those services they will go elsewhere.  If you try to fake it, they will know and be most displeased.  The later, it doesn't matter so much as long as they get good results and think it was their own wisdom that brought them forth from the ground.
Either way, it is results that matter.
Now begins my diatribe which so far has inspired much ire:
Would you jeopardize your good practices just to suit a client? If you are staunchly in favour of XP, sell it! If they want you to use a methodology that you strongly disagree with. Tell them that. If you can't come to a consensus, there should be no deal.
Do I tell a baker what grain to use? How hot to have the ovens? Hell no.  If I say I want poppy seeds on the buns I don't care how they are put there so long as they are there. Dp I select a baker based on his methods, or on how damn tasty the bread is? Letting a non programmer tell you how to do your craft is just plain bad.
If you are trying to extol the virtues of XP then be upfront, pitch the cost-benefits and ROI.  Show them why it is better for them in terms of developer efficiency and defect reduction.  If you are working for non-programmers, you are the expert, take the reigns and give advice.
If your team excels at XP and has great results you will have no problem selling any potential client on your practices.  Results matter to clients; if you can prove that you consistently produce high quality products within consistent timelines you should have no problem selling your methodology. (with some exceptions that absolutely require waterfall).