There are two types of Planeswalkers now, spark and bladder. Sparks are like Urza, being able to do bascially whatever you want when you want to, have nearendless power, they also cause tears in the multiverse when they asscend. Time Spiral was about the mending of these tears, which required a planeswalker spark, or as they showed, a large amount of power channeled through a bladder, to close.
Bladders are a lesser version, and probably the planeswalkers we are seeing made into cards. They have the ability to planeswalk, but have no where near the power of a spark.
(Or that is how I recall it)
Karn was built, or rebuilt himself, to be able to planeswalk. So when he lost his spark he was still able to 'walk. Now when he expended the spark that had been passed around the ages, an (evil) essense, from the past, attempted to take over his body. I am assuming the 'heart' used to give him life (it is of phyrexian make, Urza stole it).