I‘ve setup FreeBSD and OpenBSD under KVM. Everything works great except ftp protocol. I’ve no firewall installed inside FreeBSD and OpenBSD. How do I configure Linux host KVM firewall to allow ftp traffic?

You need to configure iptables to allow all traffic to be forwarded across the bridge from KMV guest. There are two options here:

Option #1: Configure Iptables

Type the following command as root user on KVM host (RHEL / CentOS host system):

/sbin/iptables -I FORWARD -m physdev --physdev-is-bridged -j ACCEPT
/sbin/service iptables save
/sbin/service iptables restart

Option #2: Disable Iptables On Bridges

Edit /etc/sysctl.conf, enter:
# vi /etc/sysctl.conf
Append the following config directives:

# Disable iptables on bridges so that ftp will work
net.bridge.bridge-nf-call-ip6tables = 0
net.bridge.bridge-nf-call-iptables = 0
net.bridge.bridge-nf-call-arptables = 0

Save and close the file. Reload the changes:
# sysctl -p