Hmm only one difference that I have noticed is... Guys tend to use kawaii, not kawairashii when they talk. Young women also tend to say "kawaii~~!!" rather than "kawairashii~~!!" when they see something/someone pretty/cute/lovely. Maybe only older ladies use kawairashii in the spoken style, I think...

Kawaii is cute like "the puppy is cute" or "the new girl is cute". Kawairashii is more of "lovely" like "these are very lovely curtains". Also, kawaii is also more of a informal version of Kawairashii.