After 14 seasons the mystery that Kenny has been revealed, somewhat.  In the second episode of the "Coon and friends" story arc titled "The Rise Of Mysterion", Mysterion is not only revealed to be Kenny, but Mysterion himself reveals that his secret power is that "he cannot die".

He goes on to explain that he has been killed many times but each time he wakes up in bed the next day as if nothing ever happened.  The third episode of the story arc is set to air this Wednesday.  You can watch full episodes at South Park Studios right now by clicking on either graphic.