First problem is solved by Making password as XMLText and PasswordType as XMLAttribute inside Password Class, then it geenrated XML correctly. Still not getting Namespace of BSVC Inside Body Attributes:

public class UsernameToken
    {
        [XmlElement("Username")]
        public string Username { get; set; }

        [XmlElement("Password")]
        public PasswordData Password { get; set; }
    }

    public class PasswordData
    {
        [XmlText]
        public string Password { get; set; }

        [XmlAttribute("Type")]
        public string PasswordType { get; set; }
    }