SYN scan is the default and most popular scan option for good reason. It can be performed quickly, scanning thousands of ports per second on a fast network not hampered by intrusive firewalls. SYN scan is relatively unobtrusive and stealthy, since it never completes TCP connections. It also works against any compliant TCP stack rather than depending on idiosyncrasies of specific platforms as Nmap 
A FIN scan is a type of scan whose usual aim is to perform network reconnaissance. What's attractive about A FIN scan from the attacker's point of view is that the attacker sends a special signal (a tcp packet with only the FIN flag set) that tends to get past many firewalls.
Xmas scans derive their name from the set of flags that are turned on within a packet. These scans are designed to manipulate the PSH, URG and FIN flags of the TCP header.
The Null Scan is a type of TCP scan that hackers — both ethical and malicious — use to identify listening TCP ports. In the right hands, a Null Scan can help identify potential holes for server hardening, but in the wrong hands, it is a reconnaissance tool. It is a pre-attack probe