"Peter's foot is aching."
Translation:पीटर के पैर में दर्द हो रहा है।
होता है would make it a general statement. हो रहा है makes it present continuous (present progressive), which describes something that is currently occuring. It's like the saying "Peter's head is aching" and if you just used है it would be more like "Peter's head aches" or "Peter has a headache."