Do players get xp from kills that a summons does? Is there a numerical value for xp listed somewhere?

Conjuration goes up whenever you have a Summon/Bound Weapon out. Also, cast Soul Trap on a Dead Body!

You get points I believe when you summon a creature in the middle of a battle

You don't have to cast a conjuration spell while in battle.
You do however have to have a conjuration effect working while in battle.
What this means is that you can cast a reanimate,conjure atronoch or bound weapon before entering a battle.
As long as those spells are still active when you enter the battle you will gain experience to your conjuration skill.

This can be handy when you know you are about to enter a fight,cast your conjuration spell of choice before the enemy see's you then start the fight.

This casting of spells also works for other spell schools like alteration,for it's mage armor spells.
A rule to remember though is that you won't actually get the skill increase until you actually engage in the fight.

I prefer to cast Soul Trap on Dead Bodies. I Mastered Conjuration in like 30minz doing that.

But once you reach 100 conj you can't gain any xp from conj? True?

Since you only level from skill increases, yeah, you don't gain any conjuration xp after 100. You will need to raise your other skills to level up.

You don't gain xp from kills period (by summons or yourself). You gain xp when a skill, any skill, increases. And the above posters are correct, your conjuration skill will increase if you have a conjuration spell in effect when in battle. I've had a summon out prior to battle and part way thru the battle gotten the notice that my conjuration skill increased.

Yes.

I see. So I will farm skills instead of mobs. Tx for clarification and tips.

Don't think that is completely correct. That's how I raise alteration and Conjuration. I always cast i.e ebony flesh and conjure a flame atronach and I, often, get the skill increase when not in battle.

Are you sure there isn't an enemy around that notices you?
I don't get the skill increase until there is an enemy around and they are a red dot on the compass.

Yes, I'm pretty sure. I distinctly remember a couple of times when my alteration skill has gone up in Whiterun and the College of Winterhold. Only spell I was using was 'Ironflesh'.