Answer

Mahavir Swami is both:

• Arihant Bhagwan during His embodied state after attaining Keval Gyan.
• Siddha Bhagwan after attaining Nirvana / Moksha.

Explanation

When Bhagwan Mahavir attained Keval Gyan, He became an Arihant because He had destroyed the four destructive karmas and was still present in the body.

Later, when He attained Nirvana at Pawapuri, He became a Siddha Bhagwan, completely free from all karmas, birth, death, and body.

Takeaway

So, while living after Keval Gyan, Mahavir Swami was Arihant Bhagwan. After Nirvana, He is Siddha Bhagwan.