VaR - Value at Risk is the potential loss that might arise while dealing with securities for a stipulated timeframe. VaR margin is needed to cover up for the losses occurring due to uncertain risks.
VAR margins are covered for a single day for liquid securities and 3 days for illiquid securities.
ELM - Extreme Loss Margin (ELM) covers the potential loss in scenarios that go beyond those predicted in the 99% value at risk estimates that are used in VaR margins. It is collected or adjusted against the total liquid assets on the member on a real time basis.
To learn more about VaR and ELM you can refer this BSE link.
AdHoc - This is a type of margin that is collected by the exchange from the members who have massive outstanding positions. It is also charged on volatile scrips on adhoc basis and varies according to the gravity of the risk.