I think the problem is with emp_id column. You are considering it as String but in the Database you might have its datatype as Integer/ Number.
If the emp_id column is numeric then use -
Convert.ToInt32(lvVar);
in your code as-
SqlDataAdapter adp = new SqlDataAdapter("Select First_Name, Last_Name, DOB, Designation, Department, Join_Date from Employee where Emp_ID = " + Convert.ToInt32(lvVar) + "", cont);
Otherwise rest of the code is correct i think...