Which differences can be attributed to character development, which to change in conceptEdit
The manga was the first continuity to come along, so obviously it contained a lot of idea sketches, changes, and developments. The original basis of the series was a goddess coming to live with a man, and eventually as time went on it was clear that Mr. Fujishima had other ideas that came to mind; even if his new ideas clashed with his old ideas, he simply ignored those discrepancies in continuity and forged ahead.
The OVAs are the next continuity. As mentioned previously on other sections of this site, the OVAs rewrote the beginning of the story as if Mr. Fujishima knew exactly how his characters would end up currently. I would say these changes are due to the fact that the vision is now different; it is no longer simply about a goddess living with a man, but also a conflict between worlds and rules (as further shown in the movie).
The TV series is the most recent. It seems to me that they attempted to redo the OVAs and take them to the next level; all of the character appear as they currently appear in manga canon, and have their current personalities. However, the concept is the same; it is the characters who have evolved.
Is the OVA a summary of the manga or a whole new incarnation?Edit
I would say a summary as well as a new incarnation, as Mr. Fujishima now knows where he's going with the story. He simply rewrote the very beginning of his story to fit his new concept, so it's more like a Director's Cut of the first few stories. It is also a summary, because the OVAs take place over a very long period of time, about a year. Just between the first three OVAs the time that has passed has been about nine months.