bibtex to xml conversion can actually mean two things, the first being the most obvious probably, would be export of citations/references from the BibTex document and save these in a separate, XML format. This is best to be done with a tool designed for this purpose, for example JabRef.
Another way to look on this is just simple export of BibTeX document to XML format, which is certainly doable with one of the many available TeX document preparate solutions.