The default VNC startup script (~/.vnc/xstartup) doesn’t include any commands to start standard desktops like KDE or GNOME. It contains commands to start primitive TWM as window manager and one xterm terminal as application. The following will be the typical startup file for VNC in Fedora systems.

[neo@neo.techpulp.com ~/.vnc]$ cat xstartup
#!/bin/sh

# Uncomment the following two lines for normal desktop:
# unset SESSION_MANAGER
# exec /etc/X11/xinit/xinitrc

[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
xsetroot -solid grey
vncconfig -iconic &
xterm -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" &
twm &

You can change the line which starts twm as default manager to a command which starts standard desktops like KDE, GNOME etc. startkde command can be used to start KDE desktop or gnome-session command can be used to start GNOME Desktop.

The configuration file (~/.vnc/xstartup) will be as follows to start KDE desktop by default.

[neo@neo.techpulp.com ~/.vnc]$ cat xstartup
#!/bin/sh

# Uncomment the following two lines for normal desktop:
# unset SESSION_MANAGER
# exec /etc/X11/xinit/xinitrc

[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
xsetroot -solid grey
vncconfig -iconic &
xterm -geometry 80x24+10+10 -ls -title "$VNCDESKTOP Desktop" &
startkde &