Rather than being a normal animal or beast, your companion or familiar hails from the heavenly realms.
Benefit: Your animal companion, familiar, or mount gains the celestial template and becomes a magical beast, though you may still treat it as an animal when using Handle Animal, wild empathy, or any other spells or class abilities that specifically affect animals.
Does Celestial Servant change my companion creature’s HD, saves, skill ranks for changing its creature type?
No. All it does is change the creature’s type to “magical beast” (which makes it immune to abilities from other creatures which only affect animals) and give it the abilities of the celestial creature simple template (which does not change anything other than what is exactly specified in the template).
Pathfinder Roleplaying Game Advanced Race Guide © 2012, Paizo Publishing, LLC; Authors: Dennis Baker, Jesse Benner, Benjamin Bruck, Jason Bulmahn, Adam Daigle, Jim Groves, Tim Hitchcock, Hal MacLean, Jason Nelson, Stephen Radney-MacFarland, Owen K.C. Stephens, Todd Stewart, and Russ Taylor.