The fairy myth has more Germanic influence while the pixy myth has more of a celtic influence.
Today's definitions are much more vague and so both fairies and pixies are labelled one and the same by many. Much like going into an Irish store and finding Scottish merchandise. People simply no longer know the difference.
I think in Tinkerbell's case, she really is a pixy, as she is quite mischievous, which is a pixy trait.
In the end though, today's depictions of both pixies and fairies is nothing at all like the original folklore myths.
Talk about completely off topic in this section of the boards though! LOL