$A$ says "Atleast one of us is a knave".
Case $1)$ Suppose, $A$ is knight
Since, $A$ is knight, So, he always tells the truth. So, here , "Atleast one of us is a knave" will be a true statement. Since, $A$ is knight, So, $B$ will be knave.
Case $2)$ Suppose, $A$ is knave
Since, $A$ is knave, So, he always tells lie. So, here , "Atleast one of us is a knave" will be a false statement. It means both $A$ and $B$ are knights which is contradicting because $A$ is knave. So, It is not possible here that $A$ is knave.
So, Conclusion :- $A$ is knight and $B$ is knave.