Is the XP earned shared with the henchman?

I don't think so...The henchmen level up whenever you do.

and the Henceman is always one level under you.

I do remember something that you get a little less XP if you have a henchman, but they indeed level when you do and are one level lower.

Having a henchman, familiar, summoned monster, or animal companion (or all of the above) affects the Challenge Rating you pose. As a result, it does affect your experience points, but not always for the worse. You get harder monsters to fight in many encounters, which can easily offset any loss from having the additional helpers.

Unless the party member is significantly higher or lower level than you are, you shouldn't see too much of an effect. Where you can get hurt is at the low levels where the CR of you familiar/animal companion is much higher than yours. However, that shouldn't apply to any henchman situations and stops being an issue in the latter half of chapter one anyway.

Aloha
Vormaerin

Thanks Voermarin.
I thought they lowered the XP. However, I was unaware that the game automatically adjusted the strength of the monsters for you.
It's too bad that summoned monsters lowers your XP. While henchmen, animal companions, etc. lower XP awards in third edition pen and paper, summoned monsters never have in the games I've played.

Trur Kull, and the converse was also true. you didn't get xps for killing summoned monsters. I wonder if that's the case in NWN?

Yeah, but you also have to consider that in pnp a monster summoning spell lasts 1 round per level. In Neverwinter Nights, it lasts until the monster is killed or you rest. That's a lot more powerful. And pnp Druids can't instantly replace their dead animal companion in the middle of a streetfight, either.

Aloha
Vormaerin

I thought they lowered the XP.
Well, they did. You only get 10% of what you would get in PnP. But that's because you slay way more than in PnP and it also goes a lot faster. This is scalable by the way in the toolset. The 10% counts for the official SP.

However, I was unaware that the game automatically adjusted the strength of the monsters for you.
Does not always happen, but you can make a trigger that spawn an 'easy', or 'impossible' encounter. The game will then check how cool you are and reacts by spawning the appropriate creatures.