It is its own version. The paladin's version bypasses DR, does extra bonus damage vs certain creatures and gives the paladin a bonus to AC vs the target of the smite.
The Celestial Template does not bypass DR, nor does it give a bonus to AC. It only does 1 point of bonus damage per hit die, regardless of the type of evil creature targeted.
A paladin's smite and the smite granted by the Celestial template were a lot more similar in 3.5. Both were upgraded in the conversion to pathfinder but the paladin's was upgraded much more.
Bottom line, they are similar but not the same.
As already pointed out, it's slightly different. You can find the description in the Bestiary:
Special Attacks smite evil 1/day as a swift action (adds Cha bonus to attack rolls and damage bonus equal to HD against evil foes; smite persists until target is dead or the celestial creature rests).