DivX (v5) and XviD formats are practically identical. XVid will play DivX (v5) and DivX (v5) will play XVid.
The only major difference is that DivX (v5) is legal (and costs money or advertising) and XVid isn't. DixV (v3) which used to be common among the pirate community, is itself pirated Microsoft software.
Depending on how an XVid file was saved, the DivX codec might not recognise it. There are programs around (e.g. AbcAVI which can change the recommended codec in the file, allowing DivX to recognise the file and play it without recompressing it).