Wrong, wrong, wrong, wrong wrong!!!
Ok, since no-one is able to grasp the concept themselves, I'll spell it out for you.
Xehanort, taking on the name Ansem, shed his body and took on what he saw as a higher form, which is the entity we know as 'Ansem' in Kingdom Hearts. This form, however, was destroyed by Sora after it's combination with 'Guardian', Ansem's shed heart, or, in other words, his heartless. The soul and the heart recombined into the huge airship-like enemy.
This, however was destroyed. A piece of Xehanort remained inside Riku, however, and he attempted to use it to take control of the boy. Riku, however, resisted, and gained control of the fragment of Xehanort, instead of the other way around, as is shown in his story in CoM.
Xemnas, who many people seem concerned about, is in fact not the nobody of Xehanort, but the nobody of Ansem, who since has taken on the name DiZ: Darkness in Zero. Xemnas was formed when Ansem fell to the darkness, from which he returned after seeing it corrupt his apprentice and friend, Xehanort.
Xenohart, Xehanort's nobody, is not seen in KH2, only Riku wearing his form. It is unknown if Xenohart still survives, or if he was destroyed utterly in the process of Xehanort shedding his body.