(No spoilers please :P)

I was playing as a Monk, hiring Tommy as henchmen, I am almost finishing chapter 1 at level 8

But I read in OLD forums that you can have slightly more exp going solo, henchemen slightly split exp..

So I decided to start solo as paladin, it’s not only a biiiiig pain, worse I feel I am not only receiving not more experience, I feel I am RECEIVING LESS! And leveling even slower!

What’s going on?

There is something called Effective Character Level. In OC some classes (mostly those spellcasting oriented ones) are receiving experience bonus at low levels. Read more: http://nwn.wikia.com/wiki/Effective_character_level

So first, what Kerebron said. Paladins will get less XP than most other classes to start because they're stronger to start. That only applies in modules where that feature is enabled (most do not use it -- the Shadows of Undrentide and Hordes of the Underdark campaigns ignore it as I recall, plus all of the community made campaigns ignore it).

In general, each henchmen, familiar, animal companion, or summon lowers your XP gain by 20%. That's multiplicative, not additive, so having three henchmen would be 0.8 * 0.8 * 0.8 = 51.2% of normal XP. However, some modules will spawn more/tougher enemies based on the total "power" of your entire party -- so if it spawned twice as many enemies for that party of four you'd technically wind up slightly ahead on XP. Most don't quite that far, though, and some modules just have set enemies that don't scale whether solo or not.

Of course, trying to do a group module solo can be massively difficult or impossible and ultimately it won't matter much in the end (having a group of three level 15s is better than one level 17). The original campaign is easy enough that you'll be more than fine solo (all of Bioware's campaigns are)...but many community campaigns are of a more reasonable difficulty.

Wow! that explains it

thank you guys!