These are not opposite ends of a spectrum. Nervousness is a response to importance; we tend to feel nervous about an important event, good or bad. Confidence is a feeling of self-assurance, certainty or capability.
Nervousness says, “I hope this goes well!”
Confidence says, “I’m pretty sure it will.”
It’s likely you would feel nervous about the prospect of your viva because passing it is important. It’s possible to feel confident for it because of all of the work you’ve done, all of the things you know, all of the talent that you must have. Reflecting on all of these things can help you to find and feel confident. It’s a far more useful thing to do than to try to squash nerves.
Nervousness is about the event. Confidence is about you.