Hi,
I would say that the main difference stems from the reason the appointments are made in the first place. A "preventive" visit is made when patients are not suffering from any type of illness/sickness (asymptomatic) but are taking a "preventive" measure in maintaining their well-being and health. They are not concerned with addressing current symptomts because there are none.
A "new" visit is one that requires addressing of specific diagnosis (runny nose, heart murmur, indigestion, ear infection/pain, ....and so on). A phyisician will limit the exam to that which is necessary to diagnose and treat the reason that brought the patient in to his office in the first place.
With regards to the actual exams, there are different criteria, review of systems, patient and family history that are required during a "preventive" visit versus a "new" visit.
I know this is just a "nutshell" version of the two exams but I hope it helps! Good Luck!
*Keep in mind that both types of exam can be billed on the same DOS (modifier -25 on the "new/est" code) if criteria is met for both exams.