What is the role of an exhibitor in a research carnival? Do they need to present a paper or their achievement such as a conference paper?

A research carnival is the most informal type of academic symposium, less formal than a conference paper presentation or even a poster presentation.
Research carnivals, unlike research conferences, are designed to be accessible to the general, non-academic public and to present the findings of research in a way that is both understandable and compelling. Instead of papers or posters, participants in a research carnival present tangible examples of their research so that attendees can understand its practical implications and witness it "in action."
For instance, at a research carnival, an exhibiting robotics engineer would not present a paper on their studies, but they might present working examples of robotic devices they had built with the aid of their research and demonstrate how that research had contributed to their design and construction.