If i begin taking kariva birth control pills while on my period, are they effective immediately?

Yes: If you start taking your first pill, of a combined oral contraceptive pill, on the first day of your period- you do not need to use a second form of contraception to prevent pregnancy. Condoms are still a good idea, for lots of reasons.

Yes they are : Yes these are effective if you take at the start or during your periods as it suppresses the ovulation which happens at about mid period , they also lakes vaginal mucus thick so that sperm can not travel to uterus and harder for the fertilized egg to attach to the uterus.