For an Android app I am currently working on, I would like to be able to parse raw XML data from an RSS feed and display it. The RSS feed in question uses Feedburner for publish
You should use ?format=xml to get raw XML for a feed. Indeed, both ways of retrieving feed XML works. But not always. ?format=xml however worked on all feed I've tried, but ?fmt=xml does not. I could guess, fmt is legacy parameter, as the feeds where View feed XML button available use format.
Often, FeedBurner provides XML feed directly (don't be confused with stylesheets used to prettify RSS).