http://www.mkyong.com/java/how-do-convert-byte-array-to-string-in-java/

and then you can load string to XML with 

parseXML();

http://processing.org/reference/parseXML_.html

UPDATE:

But remember about memory! At the time of conversion you will have two copies of your file in memory: one as string and one as byte[] so if the file is super huge (dunno like 200Mb) then you will need like 400Mb in memory to do this conversion. And when you create XML structure it will as well allocate some memory. So you may want to nullify reference to the byte ;) (hahaa you guess where I am coming from :) )

I'll check out that way and maybe in some time try to build a more efficient way of doing it, however I believe I won't be having 200 MB xml files to worry about xD

Thanks!! :)