Put simply, a Pattern Library unifies a production team and a style guide unifies an organization. A Pattern Library is a tool often used by developers and designers to unify around building project components - good examples of these are Storybook and Pattern Lab. A style guide can include components from the Pattern Library but also includes other information useful to other teams such as content editors, user experience, accessibility and executives. A good example of a style guide is Carbon Design System.
A Design System combines a style guide and a Pattern Library (see above). It is both the workshop and the storefront.