Hypervigilance is when you are constantly on-guard for any danger in the environment. Being vigilant is when you are aware of your surroundings. But you become hypervigilant when you are on-guard constantly and with more intensity. You become so guarded that you are easily startled by loud noises. Traumatic events and having post traumatic stress disorder (PTSD) can make you hypervigilant for any dangers in the environment.
Paranoia, or paranoid delusions, are fixed, false beliefs that you think people are out to get you and harm you. Paranoid delusions can be seen in numerous mental disorders, such as schizophrenia, schizoaffective disorder, bipolar disorder, and depression. Paranoia can also be present in dementia, delirium, and acute intoxication/withdrawal from recreational drugs.
In paranoia, you really do believe someone is trying to harm you or kill you, while in hypervigilance, you are guarded and suspicious as you believe that the trauma is going to occur again. So in paranoia, you believe you are being pursued and harmed now, while in hypervigilance, you are anticipating the danger that someone or something will harm you sometime in the future, due to your past history of trauma. Another difference is that hypervigilance is usually associated with PTSD, while paranoia is usually associated with psychotic disorders, dementia/delirium, and intoxication with drugs.
To learn more about hypervigilance and PTSD, please watch the following video: