Artoomis, the Energy Drain special ability also adds negative levels. It's in the DMG, Special Abilities, page 75, second bullet. Also, if you have gained negative levels equal to your current level, you die. DMG, page 76, second to last bullet for Energy Drain. Last, but not least, if you are drained below first level, you die.
Gaining enough negative levels will kill you, just as being drained of enough levels will kill you. Both will drop you cold. The only difference between the Energy Drain spell and the Enervation spell is that the negative levels bestowed by Enervation automatically return after a number of hours equal to the level of the caster of the Enervation spell. The negative levels can still kill you.