It was confirmed already that both Xemnas and Ansem SoD have minds of their own independent from Xehanort and that the "heart-conglomerate" that formed Apprentice Xehanort most likely stayed with Ansem at least during the time of KH 1 to KH 2. So that means Xemnas started out without a heart like all other Nobodies, possibly with a shard of Xehanort's heart already left inside him.
Nonetheless he definitely developed an own heart for himself.
Now since Xemnas had memories from both Master Xehanort and Terra (unlike Naminé and Roxas, who had nothing) his new sense of self and heart of course developed in that direction and the Xehanort-heart shard present inside him took care of the "Xehanort"-traits being dominant in this new heart, evident by gold eyes, silver hair and how much of a prick Xemnas actually turned out to be. Nonetheless some of Terra's memories apparently still managed to snuck in and influenced him to a degree as Xemnas clearly knew of Aqua and Ven.
That's one of the things that are iffy though, the notion that Xemnas and Ansem SoD have own minds and most likely hearts would have made a much easier explanation on how they would be able to exist alongside MX without any time-travel jibba-jabba.