can you use an airport extreme with xfinity modem

You can.....but if the Xfinity modem/router is already providing routing services and a wireless network, I'm not sure what you would hope to gain by adding an AirPort Extreme to the network......unless you really need an additional wireless network or there is some special feature that you want to utilize on the AirPort Extreme.

If you elect to use the AirPort Extreme, it must be connected to the Xfinity modem/router using a permanent wired Ethrernet cable connection. Connect from a LAN <-> port on the Xfinity modem router to the WAN "O" port on the AirPort Extreme.

I don't use my AirPort Extreme as a bridge, I use it as the main wireless router connected directly to the internet via 'a certain big company's' modem. What I stand to gain by using my AirPort Extreme wireless instead of the one they provided me is peace of mind. I don't trust ISP-supplied routers, especially one supplied by 'a certain big company'. With reports all over the internet of that company having back-door access, compatibility issues and my personal experience of them not delivering on what I purchased, and then trying to up-sell me when I complain about it, I'd rather work with a known quantity. I know what security features are incorporated in the AirPort Extreme and how to set them according to my needs. If I ever read that the AirPort Extreme is less secure than their's, then I'll consider abandoning the AirPort Extreme, but I haven't read anything to encourage me to do this.