I was always under the impression it was Jenova in reality, but casting the illusion of Sephiroth (or you could view it as Sephiroth projecting his image using the powers of Jenova). That's why whenever Sephiroth comes at you before you reach his body the Northern Crater, the boss fight is always with Jenova, not a Sephiroth clone.

Also, either way even if it's just carrying Jenova's body, I think my point is still valid, because the Sephiroth clones also follow him around.