Anxious, worried and concerned can be distressing.
Nervous is uncomfortable. Nervous is reacting to the importance of a situation.
For the viva, nervous means you’re acknowledging that the exam is important. The conversation you’ll have with your examiners matters. And that feeling could be very uncomfortable.
If you’re anxious, worried or concerned then you’re not simply recognising the importance: you’re responding to a problem of some kind. Maybe there is something you don’t know about the process and it troubles you. Maybe you’re aware of some imperfection in your research or thesis and you feel uneasy about it (regardless of how big or small that imperfection might be).
If you’re anxious, worried or concerned and can name the problem that is causing that emotion then you can do something about it. Reflect, analyse the problem, ask for help but ultimately do something to lessen the burden of that problem in advance of the viva.
If you’re nervous then all you need to do is find a way to feel comfortable with the uncomfortable. Building confidence helps, learning more about the process and being prepared help too.
But fundamentally feeling nervous about the viva is OK. It’s a very common part of the process.