Packet Filter: First Generation
Packet Filtering Firewall is the first Generation of firewall. Here each and every packet is scanned or inspected when traffic goes out through the gateway devices. If any packet matches the set of access control list action it allows or drops or rejects.
Packet Filter Firewall filters each packet based on the information in the packet like the protocol, i.e. TCP/UDP and the Port Number.
Stateful Filters: Second Generation
Stateful Filtering Firewall us the Second Generation of firewalls. Here instead of scanning each and every packet it scan the series of packet. This kind of Firewall is mainly used to detect and prevent DOS attacks.
Application Layer: Third Generation
Application Filtering Firewall is the Third Generation of firewalls. This firewall does not block traffic but ensures that all the traffic passed by this firewall is clean and safe. This Firewall has UTM features enabled by default. This scan traffic for antivirus, antispam, web filters to avoid unwanted websites, Application Control to block unwanted firewall. In other words this firewall can acts as Proxy services.