GIFV is not actually a distinct file format, but it is an MP4 or WebM video that is presented via a HTML 5 video element or an Adobe Flash element with options set so as to make it work similarly to an animated GIF (i.e. endless loop with no video controls visible). This is created by Imgur when a user uploads an animated GIF, and is served with a .gifv extension (though the document sent to the browser is actually HTML). Note that if you try to save it out of your browser you're not saving the actual animated image, since the HTML file references remote elements on the Imgur server. This is not really a format to store and preserve animated graphics yourself, but one for a website to present them.
You can download an animated GIF by deleting the "v" at the end of the URL.