Thanks for your question. There is honestly no difference between "excuse me" and "pardon me". The phrase "I beg your pardon" could also be used. "Excuse me" seems to have turned into the more popular phrase of the three in the U.S. recently, but neither one has a connotation that is more polite or less polite than the other. On the other hand, because it seems that most people use "excuse me", the other phrase "pardon me" might come across to some as more formal. In Emily Post's Etiquette book, she suggests using either option with no difference--as long as one is used!
One other slightly nit picky thing--while you can use the two interchangeably, I tend to use "excuse me" for instances where I am physically infringing upon one's space, when I have to walk by someone or the like. I use "pardon me" most of the time when I ask someone to repeat what they said. That's just me though. Politeness is appreciated in most forms, and whichever you choose, you'll be in good shape. Best wishes!
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