Great question and I just researched this myself. Xfinity does not yet have a native app for AppleTV (that I know of). They are currently in beta on Roku devices...but who wants to use those. XFINITY Stream Beta App on Roku FAQs
While I bet there will be an Xfinity app available at some point (perhaps even later this year) I just went through a conversion so I could run ALL TV on my AppleTV...and it's awesome. I ended up going with Playstation Vue for TV (also tested Hulu live TV) as it seems to be the most powerful and allows concurrent streams. Just updated my Xfinity account to internet only (and increased the speed in the process).
Now, it's a great experience on every TV in the house. Each Apple TV has Playstation Vue (for TV), Netflix, Hulu, Amazon Prime, and HBO Go. Easy enough to add more or less apps as needed/wanted. Since AppleTV only costs once for the hardware, there is limitless expansion. I have them installed on everything from a 39" LCD guest room TV, to an OLED, to a 120" 4k projector in our media room. Loving the new solution.