Cash flow mapping is the most precise method of the three because we map the present value of the cash flows (face amount discounted at the sport rate for that maturity) onto the risk factors for zero coupon bonds of the same maturities and include the intermaturity correlations.

Principal mapping includes only the risk of repayment of the principal amounts and considers the average maturity of the portfolio.

Duration mapping involves the risk of the bond being mapped to a zero-coupon bond of the same duration. Duration mapping uses the duration of a portfolio (the duration measure is only an approximation) to calculate the VAR.

There is no such thing as convexity matching.