The GRUB's menu is automatically shown if you have multiple OSes installed. Since you did not say what release of Kubuntu, I'll assume it's using GRUB2.
To hide the menu on boot, open your /etc/default/grub
for editing as root. Comment out this line:
by adding a # character at the start of the line.
Now uncomment these lines by removing the # character at the start of the line:
Run update-grub and you should be good to go.