One of the reasons these terms are blurry is that personalization is a relatively new concept in some areas.
Personalized medicine is new. It uses genetics to tailor a treatment that has been customized for each patient.
Personalized learning has been around for a long time. From the teacher’s perspective, personalization means designing learning experiences that provide students with choices they can make depending on their own needs.
A customized site can provide personalized content. In content marketing, personalized content can help sell your products or services based on people’s web interactions. A good example of this is Amazon “noticing” what you are buying so they can personalize their front page.