A partner manager is there to maintain relationships. What's the difference between the partner manager role and that of a project manager?
A project manager's goal is to ensure project closure. A project manager is teleological. and consequentialist The project manager's worries are scope/schedule/quality.
I don't work as a partner manager, but I suspect that the role is far more open ended. Partner managers should, I believe, work to sustain and improve a partnership. A partner manager's worries involve competition, competitive advantage and trust.
While researching the question, I found "What makes a good Partner manager", which has this excellent quote,
Roles Can Vary
Definitions are tricky, because while some roles have well-defined boundaries, other roles mean whatever the companies want them to mean. I've known an executive during the dot-com bubble whose official title was "Senior Web Monkey," and known at least two "Senior Vice Presidents" at major banks who were hands-on managers of IT departments but had only one person reporting to them. Your mileage may vary.
Contrasting Role Definitions
With that said, a Partner Manager is often a sales-channel manager who manages the relationships between a company and their channel partners. I'm not truly sure that "partner" is really the right term, as it's often management-speak for "preferred resellers."
More formally, Wikipedia defines business partners as:
A project manager, on the other hand, is primarily responsible for project process. Wikipedia defines the typical responsibilities of a project manager role in this way:
Both Roles Include Managing Relationships
Both roles are similar in that they involve managing relationships; the differentiator is in who the relationships are with. However, the roles are also fundamentally different in their organizational purpose, and require different deliverables from each role.