You don’t have to feel one or the other for your viva. In fact, it’s likely that on the day and in the time leading up to your viva you could feel a whole range of things.
Nervousness comes from feeling the importance of viva day.
Confidence flows from feeling sure of your ability, knowledge and contribution.
The viva is important, so of course you might feel nervous. But you’re also capable, knowledgeable and someone who has done the work. As a result, you can feel confident.
It’s not either-or. You may feel nervous and confident: perhaps a little uncomfortable from the event itself, but hopefully sure of your place in the process.