They're both benzodiazepams.
Xanax has a short half life (4 hours). I believe Valium has a longer half life. Klonopin/clonezapam has a longer half life (about 8 hours), so that might be more similar.
I believe Valium is the mildest/least powerful form of diazepams - these other two are stronger. You can always cut a pill in half or quarters to get an equivalent dose.
If you're a fast metabolizer like me, you'll do better on the longer acting varieties. I've taken both Xanax and clonazepam and do better with clonazepam.
This tapering chart has info on equivalents when switching from one type of benzo to another.
http://www.benzo.org.uk/manual/bzsched.htm
The benzo tapering group Benzo Buddies or the Surviving Antidepressants forum may also have helpful info.