Paw licking by your Chihuahua could be as a result of many factors among them stress, parasites or even allergies. Sometimes idleness on the part of the dog may be the cause. You need to thoroughly clean the dog's environment just in case this is as a result of fleas as well as make the dog more busy and occupied by increasing games and other play toys. Read more on http://www.dogpawlicking.com/.
Check out the issues the first post stated, if it's none of those, then try spraying apple bitter on his paws. He should stop then. You can get it at a pet store and it is supposed to taste nasty, therefore, deterring your little pooch from the bad habit. Good luck.