2 ways to do this
if your tv has multiple inputs and an audio output its easy if not there is a way but its more complicated
if your dvd and xbox have HDMI (link to inexpensive hdmi cables)
outputs pick up some cables on ebay and use that to go directly to the TV then use the audio output of the Tv and run it to any input on your receiver EXCEPT PHONO. I use video 1 since its easy to remember.
all you do is set your receiver to video 1 and then set your tv to the input you want to watch ie: video 1 for xbox, video 2 for the dvd
this will also work if you use component inputs on your tv (the red, green blue ones)
Now the harder way to do it.
Your receiver has high def (red, blue, green) component inputs
plug your dvd, xbox and cable box if you have one into the 3 video inputs on your receiver and teh TV audio can be plugged into the CD or aux input.
here is the issue.
running both the audio and VIDEO signal through the receiver may degrade your video quality based on the type of video processor that your receiver has.
secondly lets say your dvd doesn't have component outputs and you have to run s-video cable instead then you have to run multiple monitor outputs to your tv as the receiver wil not "upconvert or downconvert" the signal from one format to the other.
Newer higher end receivers will do the conversion but usually not until you get into the $500 and up price point.
when i do installs I try to ,ake it so that people have to push the fewest amount of buttons. Most people know how to switch the input on the TV but then get confused if they have to also start switching inputs on secondary devices such as receivers
hope that helps