Guyver I was called Guyver I because it was the first unit to be activated and personally I think that all 3 Unit G(s) are the same and that the power difference and shape difference is because of the first fusion with the owner. If he is a weakling, then the armour may turnout a little weaker, etc.

Which in the case of Sho, his was the "weakest" of the three and as Agito has plans for world domination his was the strongest.

Gnerally, Guyvers have a few different attacks and abilities, such as the very powerful Mega Smasher, Sonic Blades and nested at their heads is a lazer firing module that was meant to protect the Control Metal if any danger arises.

From Guyver I we collected the most amount of data since the focus of the story revolves mainly arounf Sho.

The Guyver units are able to regenerate themselves because when they fused they continuously records the state of which the host is in and will recreate the last best battling state of the host if any part is damaged. This however does not include the Control Metal itself from being destroyed.

Sho for instance have been complete destroyed, had his brains ripped off and also almost lost his arm in a few battles but the control metal was able to regenerate him. ( Lucky guy :) )

Something special to Guyver I is that he was the original "creator" for the Gigantic Armor.

Some pics below from the Movie