This question was asked by my 8 year old son-- Who are the parents of Mahavishnu? He further added to put this question in stack exchange. Although I told him that Lord Vishnu manisfests himself he is the creator presever and destroyer he said we are born to mother. Likewise Mahavishnu also will have partents like me. Explore and tell.
It's Brahman (Not the Brahma. Brahman/Brahmam is also known as the Parabrahma) that only exists. Vishnu, Siva, Shakti, Brahma all are just different forms of Brahman. We cannot explain or define Brahman and if we can, then it's not Brahman.
So coming to your question, it depends on which branch you follow. The origin of Mahavishnu is different on different texts.
If you go by Shiva purana, Siva created Brahma and Vishnu. Siva was in the form of a pillar , when brahma and vishnu were fighting over who is more powerful.
If you go by shrimad bhagvata, Mahavishnu manifested himself.
There is many other concepts in Hinduism like, Swayambhoo Manu, Prakruthi- Purush Samyog etc..
It's up to you what you believe and what you have to follow. You can even think deeply (like a state of meditation) and can create your own concepts. The final truth is that only Parabrahma exists ( and only it's real) and whichever path we choose/take, it will lead us to him once we realize ourseleves.