What are XP?
They are experience points.
What is an experience?
www.dictionary.com defines experience as
1) The apprehension of an object, thought, or emotion through the senses or mind: a child's first experience of snow.
2)
a. Active participation in events or activities, leading to the accumulation of knowledge or skill: a lesson taught by experience; a carpenter with experience in roof repair.
b. The knowledge or skill so derived.
3)
a. An event or a series of events participated in or lived through.
b. The totality of such events in the past of an individual or group.
So when questioning awarding experience points the only fair questin is was this an experience for the character?
If it fits
any or all of the above definitions then it is an experience and deserves experience points.
Does the paladin walking through the traps have an experience?
Definition 1: I would say that yes the paladin has this. Maybe he's walking down the hallway and the first trap goes off, he doesn't get hurt but he might next time and is therefor more apprehensive.
Definition 2a: Yes, the character triggered the traps and has gained the knowledge that he shouldn't go dungeon delving alone because next time the traps might hurt.
Definition 2b: Yes, but it will always agree with 2a
Definition 3a: He set of three traps, each creating an experience that he lived through correct? Then yup he had an experience.
Definition 3b: Life reaching and therefor barred from discussion of a sigular event.
The encouters with the traps meet every criteria of an experience and you're saying that he doesn't get experience points? That doesn't make any sense.
The guy gets experience points for surviving the experience of the traps.
NOTE: I am not saying he deserves full experience simply that he deserves some experience. What I would probably do is calculate the amount of experience he would have gotten as part of his original party and award that to him. i.e. Party Size/xp
Here is a fictional transcript of me and my dm if I was the paladin:
DM: You don't get any experience for the traps because they stood no chance to harm you.
Me: The rest of the party doesn't get resurected because realizing how foolish I was to take a class with immunities I commit suicide.
DM: You're character would never do that.
Me: He is and on top of that I vote that you're not longer the DM because if you weren't going to award experiece because my abilities negated the traps abilities then you should have changed the abilities of the traps on the fly. You're a crappy DM who just shafted my character.
DM: Woa wait a minute.
Me: All the other idiots died but I lived so I get no XP for the traps but if they lived I would have gotten XP right.
DM: Well, yeah
Me: So for being the superior character you are penalizing me because of class abilities.
DM: I'm not penalizing you
Me: I jab my sword through my throat everyone roll new characters and I'll DM from here on out.