What is the main cause of low pcv or low blood level?

Do you mean low mcv?: Low blood with low MCV is associated with iron deficiency which could come from heavy periods, low stomach acid, or parasites.

Blood loss: Adult anemia are more likely due blood loss like in menstruation, chronic GI bleeding. Younger people may have genetic causes like thalassemia and sickle cell anemia. Other serious causes include malignancy.