What KPIs/metrics do you use for Products under MVP development?

I'm having a bit of trouble following.

If something is still under development, then you can't get user metrics since it's not available. There are some actions that can help you understand what to build, but it seems like you're interested in validating what has been built.

I would recommend getting to the smallest releasable chunk of value quickly. It could even be a single feature that can be released on its own. Then, releasing that feature and applying the same metrics that you'd use on something in production. Getting an actual product in front of stakeholders for feedback and gathering data is the best way to measure value.

How do you guys measure value generated/success if a product is under MVP development?

If a product is in production already we're using usage rate, customer satisfaction, etc.

I want to hear your thoughts for products w/c are in MVP development.

Is the hypothesis behind the MVP proven or disproven?

Value is determined by the stakeholders.  Stakeholders can be internal or external individuals.  The metrics you mentioned for "in production" products reflect that.  So, how do your stakeholders view "value generated/success if a product is under MVP development"?  They are the ones you should be asking.