What Key Performance Indicators (KPIs) are used to measure DevOps?
I don't think there are any "universal" DevOps KPIs.  For example, velocity is great, unless it's not a key driver for your business.  Amazon cares a lot about velocity because they have a massive retail operation.  That's less important for a small app with 100 users.

This begs the question: how do you select the best KPIs relevant to your business?  That's a research and discovery process that involves your entire Enterprise.

What do you care about?


Quality
Reliability
Maintainability
Velocity
Process Improvement
Service Levels


What keeps your business stakeholders up at night?  What determines whether you make money this quarter or not?  The list above might include some of those things, or it might not.  Make your list, then figure out how to align incentives across every department to achieve them.  

Incentives drive behavior, so decide collaboratively on SMART goals.  Pick two or three items off your brainstormed list, and start a measure/fix feedback cycle for each.  Don't pick too many at once- you're more likely to succeed by focusing hard on two or three things.