i have heard 2-3 things about it.
1) the arab origin
2) the profession based origin
If the 2nd then it is not a family or tribe with common roots. Because some beleiove Paracha comes from the persian word parcha meaning cloth. Many parachas especially those in NWFP are involved in textile and leather good products and have extensive networks over seas. Hence different parachas may have different regional origins but share a common family surname: they could be mongol, turk, persian, punjabi etc...
3) the afghan orgin: some say that original parachas spoke a north afghan turko-persian dialect called parachgi and later they settled down into south asia in pakistan
4) local to eastren afghanistan and northwest pakistan. A language known as Parachi is spoken by a few people in a small geographic area between pakistan, afghanistan. It is an ancient langauge related to pashto, yaghnobi and other iranian langauges.
I personally favor on what I know the 2nd hypothesis.
