Nmap Scan Explanation

-sS | SYN Scan
    | Never Establishes full TCP connection
    | Does only first step of Three Way Handshake and gives output according to the SYN/ACK response by the target
    | If target sends 'RST', which stands for reset, it means that port is closed. Else it gives response filtered or other results.

-sV | Discovering Version of Service
    | We can decide it's intensity between 0 to 9. (Default 7)
    | E.g. nmap -sV --version-intensity 9 <ip>

-Pn | Already explained there