PFFS is a private fee for service plan. There is no contract between the medical provider and the insurance company other than the underlying Medicare rules. You can use any provider who accepts Original Medicare payment, who is willing to treat you and accepts the plan’s terms and conditions. The provider can decide on a patient-by-patient and a visit-by-visit basis at the time of service whether to treat you. Emergencies are an exception to these guidelines.
PPOs are Preferred Provider Organizations. There are contracted with certain medical providers and prefer that you use them. However, you may go outside of the provider list and still have coverage without a referral. This, however, may result in additional costs for you.
HMO’s are Health Maintenance Organizations. The insurance companies restrict your choice of providers to those they have contracted with. In addition, a referral from your primary care physician is generally required.