Is Reverse Hollowing Permanent?

Hollowing can be temporarily cured by paying the Statue of Velka to reverse hollowing, or by consuming a Purging Stone. To remove the Dark Sigil, and hollowing, permanently, the player must give the Fire Keeper Soul to the Fire Keeper, and then pay her a number of souls relative to the player's level of hollowing.

What happens if I reverse hollowing?

Reversal of hollowing allows players to summon other players and invade worlds; kindling increases the maximum amount of Estus Flask charges a bonfire provides. Humanity is also needed to level up in the Chaos Servant and Darkwraith Covenants.

Do you lose levels if you reverse hollowing?

Reverse hollowing is temporary and will come back when you die a lot. In neither case are the levels affected though.