In the United States, a CV is almost exclusively limited to professions in academia, science and medicine. In the case of academia, the main focus of the CV should be to identify yourself as a scholar. The added length, should be filled with proof of your skills as a teacher, scholar, and your knowledge of education. A CV should be thought of as a living document, and it will change frequently based on the updates in your education and career.
A resume is the representation of your professional self. It is a concise compilation of your educational and professional experience, as well as the skills that make you desirable for the workforce. It is what you use to sell your merit and skills to potential employers.