I see it slightly different from Tav.

In KH my impression is this:

The body: simple, the physical existence
The heart: from what the Nobodies say, it is definitely the emotions perhaps also consciousness too as Kairi without her heart was little more than a shell.
The soul: I'd say it defines your existence, not whether or not you are alive. I think it is some greater thing that carries from life to life.

I say this about the soul since Ven looks a lot like Roxas (his visit to Destiny Island is perhaps a foreshadowing). Now, we know that Nobodies are simply body and soul. This is why they Nobodies differ from their original in looks, the heart is not there to form the consciousness, to mold the body different from the soul. Since Nobodies are just body and soul, the only thing there to mold the physical form --the body-- is the soul. Since KHI was about Heartless, KHII Nobodies, we can assume BBS is going to be about souls (obvious, I know). To put this all together, I think perhaps Sora is a reincarnation of Ven. The fact that Roxas and Ven look so much alike and Ven visits Destiny Island has to be more than just poor plot and character design.