Don't forget VDR with the Streamdev plugin.
VDR is the most mature of the three PVR backends. I have not used MythTV for quite some time and have never used Tvheadend, so it is difficult for me to compare accurately.
VDR natively supported DVB-t/s/s2/c ever since it started with the ability to record more than one TV channel from a single DVB multiplex at once (multirec I think it is referred to as). MythTV took a long time to support DVB with multirec and hence adoption by DVB users was slow. All 3 PVRs now support DVB with multirec I believe.
MythTV has many more features and a much larger community than the others, but then VDR and Tvheadend are smaller and it can be debated, more stable.
VDR support in the PVR-testing2 branch is very usable and TVheadend support is developing rapidly. Is MythTV based livetv supported in XBMC? I don't see it as an option when setting up livetv in the PVR-Testing2 branch.
VDR does support cam although I have never used one in my VDR setup. You will need to ask on the VDR forum if the DVB stream is decrypted before it is sent to XBMC via the Streamdev plugin.
Look here for a pic of the EPG available from VDR:
http://forum.xbmc.org/showthread.php?tid=45314
I am in danger here of starting a flame war centered on which PVR is best. Each PVR has it's own dedicated community who swear their PVR is the best.