My New Oxford American Dictionary blames “complex historical reasons” for English having two past participles of prove: proved and proven.
The verbs are the subject of usage notes in dictionaries and preferences in style guides. Garner’s Dictionary of Legal Usage claims “proved is the universally preferred past participle of prove.” But in edited usage, you are just as likely to have been proven innocent as proved innocent.
Both forms of the past-tense verb appear in Middle English; the American Heritage Dictionary says proved came first. Proved won the standardization battle, and proven survived (or perhaps reappeared) only in Scotland. The Merriam-Webster Dictionary of English Usage says proven has slowly worked its way back into the language over the past several centuries. Now, most dictionaries list the words as interchangeable.
For the adjectival use, the Oxford English Dictionary says proven is “the usual form in Scottish English,” “the preferred form in current North American English,” and “now also more frequent than proved in British English.”
As a verb, proved is still preferred by the Associated Press Stylebook, which says “use proven only as an adjective: a proven remedy.” If your style guide doesn't specify, you are probably fine using either past participle form.