If you use Fester on an undead, does that mean it get's hurt by healing spells less?
Fester seems to block healing, not stop damage. Cure spells specifically (and most healing spells in general) do damage to undead. This isn't the same as doing reverse healing. A literal reading of the text would indicate that it does not function in the manner you're describing.
School necromancy; Level inquisitor 3, witch 2Casting Time 1 standard action
Components V, S, M (rotted meat)
Range close (25 ft. + 5 ft./2 levels)
Target one living creature
Duration 1 round/level or 1 round; see text
Saving Throw Fortitude partial; Spell Resistance yes
Necrotic energy permeates the target, blocking healing abilities. The subject gains spell resistance equal to 12 + your caster level against effects that restore hit points or grant temporary hit points. In addition, any healing provided by effects that ignore spell resistance (such as fast healing, regeneration, and some spells) are halved. If the target succeeds on a Fortitude saving throw, fester lasts only a single round.
School conjuration (healing); Level bard 1, cleric 1, druid 1, paladin 1, ranger 2Casting Time 1 standard action
Components V, S
Range touch
Target creature touched
Duration instantaneous
Saving Throw Will half (harmless); see text; Spell Resistance yes (harmless); see text
When laying your hand upon a living creature, you channel positive energy that cures 1d8 points of damage + 1 point per caster level (maximum +5). Since undead are powered by negative energy, this spell deals damage to them instead of curing their wounds. An undead creature can apply spell resistance, and can attempt a Will save to take half damage.