I don't know when it happened, but Xfinity has released the "full" Xfinity Stream app for the Apple TV. It works on my Apple TV HD and I was able to install right from the Apple TV app store.
I installed the app. Upon first opening, you're greeted with the login page, which included a 3D barcode that took my iphone right to the Xfinity login page. About 10 seconds later, the XStream app on the aTV was fully logged in.
There was a disclaimer page saying that currently, Xfinity does not charge a fee for the use of the stream app, but they reserve the right to do so in the future. I believe this is boilerplate and recall that when the Roku version of the app was released a few years ago, they had the same notification.
As for the app, I have full access to Live TV via the grid. I also have access to recorded programs on my DVR. I have an older DVR that has on-board storage and I believe that this app can access those recordings. I don't think it has full access to On-Demand PPV content.