How do you break down responsibilities and KPIs for product launches between product management and product marketing?

If you are lucky enough to have a PMM partner, then I would create a very transparent RACI chart which showcases who is:

R - responsible

A - accountable

C - consulted

I - informed 

about the various pieces of work. In my experience, depending upon the size of the team/company roles and responsibilies realy vary between product and PMM when it comes to product launches. Here is just one example of what could work:

Product Management

- Responsible for managing launch ready engineering and design work

- Responsible for ensuring stakeholder buy in 

- Responsible for iterating on any user research or user feedback

- Responsible for engaging product marketing early on and working backwards from their timelines

- Responsible for the overall KPI

Product Marketing

- Responsible for creating launch material for end user purposes

- Responsible for sales enablement

- Responsible for training, communication plans and go to market strategy

- Depending upon the specific product/feature area, could be responsible for activation, acquisition, # of eyeballs seen

These two functions have to work hand in hand in order to succeed

When it comes to product launch, I make Product Marketing responsible for KPIs related to enabling Sales, Marketing, and Customer Success. Proroduct Management is then responsible for owning all KPIs related to ensuring Sales Engineers and Support are enabled. 

I also expect Product Marketing to hold cross-functional launch sessions with these key groups to ensure all items from the launch plan are completed prior to launch. I would encourage PM to develop a similar launch checklist so that PM and PMM are aligned on who does what. If your PMM does not show you a copy of the launch plan, I would ask for it and ensure you two have discussed what they're doing.

It's also important for PM to understand what specific tiering your launch is in (e.g. Tier I, II, III) and from there to have a good understanding of waht actual marketing collateral PMM is going to create for your product. Typically PMM will have defined Tier I, II, and III in your organization and what each will provide the product. If you haven't read this, ask your PMM. If your org has not defined this, then I'd ask PMM to do this for you. 

Product Management is responsible for Product strategy but that needs to align to the Product market fit which is defined by the GTM strategy that is defined by Product Marketing. One of the key requirement is that we from product align to the GTM strategy before defining the Product strategy. This will help build high quality products that bring customer delight and high ROI. It is crucial part of partnership from Business perspective, just like how partnership between Product and Engineering is crucial from Execution perspective.