Sri Parasara Bhattar in his elucidation of Sri Vishnu Saharasranamam for the ‘eka pata’ sloka, says that Sri Krishna is half part (amsam) of Sriman Narayana. In the previous sloka, he says that the four-armed Para Vasudeva is incarnated as Kannan. He also expounded that Kannan is the primordial Lord, quoting Gita as a reference. Whose incarnation is Kannan?

Kannan is the incarnation of the primordial Lord Sriman Narayana. He is also the incarnation of Para Vasudeva.
Notes:
Sri Vishnu Puranam says the avatars of Sri Rama and Sri Krishna are elements (amsam) of Perumal.
In the commentary, when it is said that incarnations are a component, it does not imply only the power related to that component is present in that incarnation.
During his incarnations, Perumal always reveals some of his power and this is what we refer to as component (amsam). Perumal is always omnipotent endowed with auspicious qualities.