The main purpose of MVP is to test your assumptions in an inexpensive manner. But you can do the same thing with an NVP also known as No Viable Product. So what comes before an MVP is an NVP.
NVP comes in between the business canvas model phase and an MVP phase. NVP is like a sales pitch created out of the assumptions in your business canvas model. You then try to sell this NVP to your target market. Being successful in your NVP will in a way validate your assumptions even before you develop the MVP. If you are unable to sell your NVP then you need to rework on your business canvas model assumptions.
You read an article here for more understanding: https://www.linkedin.com/pulse/what-comes-before-mvp-travis-o-johnson/