What is the difference between these roles and where is the crossover? Is it possible to be both in particular jobs?
I would say that we are looking at two different levels here:
An IT manager could be a project manager but need not be one - and vice versa. What complicates matters - and insofar the question is a very good one - is that many IT related activities are managed in a project oriented manner.
So an IT manager can be a line manager but in many cases will be a project manager and a project manager manages projects - which can be IT related (or not).
So both levels are interrelated in many cases.
The PMBOK defines a project as a "temporary endeavor undertaken to create a unique product, service, or result." PROJECTS differ from OPERATIONS. There are many different methodologies and best practices that can be applied to Project Management (as well as operations). IT Management can include Project Management and/or Operations Management, but it doesn't have to include both. It depends on the organization.
This depends a lot on the culture and size of the organization. In pure delineation of duties: PMs manage projects and IT managers manage IT people/departments. Many projects have a significant IT component, even if they are not entirely IT driven projects. A strong relationship with the responsible IT manager will go a long way to ensure project success as long as you both agree on roles and responsibilities at the outset. PMs shouldn't write code and IT managers shouldn't distribute the project update report.
The need and organization of the PM role is as varied as companies and projects but you will see general trends and setups. In small organizations there is usually crossover or a breakpoint on what qualifies as a corporate project, requiring a PM. Some organizations have a separate PMO office and some have PMs imbedded in business departments. Some organizations simply appointment someone to deliver the project, without PM title, or don't even have a PM job/role.
In my (brief) experience as an IT manager, and then later in finding my replacement, I found the following held true:
Small companies usually had an IT manager (like myself) doing all the management tasks, whereas a larger company would have a dedicated Project Manager organising work and a people management focused IT Manager doing appraisals etc.
At least within my industry sector, the emphasis is that project managers "create a unique product, service, or result." (hat tip to @craig Villacorta); this excludes routine operations, and activities that are designed to generate yesterday's results tomorrow.
IT managers are focused on "IT service management (ITSM or IT services) refers to the implementation and management of quality IT services that meet the needs of the business." Wikipedia goes on to focus on the relationship with the customer, and on process improvement - things that I think are generally perceived as NOT project oriented because they are about the process, not the discrete result.
There is overlap; IT Service managmeent can involve the implementation of services (which can be managed as a project). Project Managers certainly care about quality and about managing the stakeholders/customers. They each use many of the same tools and processes.
But in summary, IT Services Managers generally involve providing ongoing services, which project managers provide management for a discrete, result oriented project.