What is the difference between hyperglycemia and diabetes?

Different stage: Hyperglycemia just means your blood sugar is high. You may or may not be diabetic. If it's chronically elevated above a certain level (>126 for a fasting sugar) then we call it diabetes.

Hyperglycemia: If blood sugar level above a threshold of 120, then it is called, hyperglycemia. The difference is, in diabetes, blood sugars tend to stay > 120 and higher if left untreated, where as in hyperglycemia, the sugars trend down within a few hours. Hyperglycemia-- is an abnormal blood test. Diabetes is a disease.