PAR 38 LED in a PAR 30 recessed can
First the good news: Electrically it would be fine as long as the PAR38 bulb's wattage does not exceed the receptacle's maximum wattage.  In the case of an LED bulb replacing an incandescent household bulb, you'll most likely be replacing a higher-wattage bulb with a lower-wattage bulb, but you should check the specs to be sure.

Now the bad news: PAR38 and PAR30 refer to the physical characteristics of the bulbs.  PAR = Parabolic Aluminized Reflector, 38 = thirty-eight eighths of an inch, or 38*1/8"=4.75".  30 = thirty eighths of an inch, or 30*1/8"=3.75".  As you can see, PAR38 has a larger diameter than a PAR30 bulb or can.  Depending on your fixture, the PAR38 bulb's connector may not even reach the receptacle, and if it does, the bulb will protrude past the fixture rather than being recessed.  Some people use extensions to bridge the gap and use the PAR38 bulbs anyway, but personally I think it looks horrible.

Also, keep in mind that a PAR-type bulb may produce more of a spotlight effect, whereas a BR-type bulb typically produces broader illumination, so you may want to look at BR30 bulbs unless you have high ceilings or prefer a more focused beam.