# xen/manifests/init.pp

[...]

class xen::boot {
    include grub
    exec { "mv /etc/grub.d/10_linux /etc/grub.d/20_linux":
        unless => "find /etc/grub.d -name 20_linux",
        notify => Class["grub::update"]
    }
    exec { "mv /etc/grub.d/20_linux_xen /etc/grub.d/10_linux_xen":
        unless => "find /etc/grub.d -name 10_linux_xen",
        notify => Class["grub::update"]
    }
}


# grub/manifests/init.pp

[...] 
 
class grub::update {
    exec { "/usr/sbin/update-grub":
   }
}

# error

Could not run Puppet configuration client: Could not find dependent Class[Grub::Update] for Exec[mv /etc/grub.d/10_linux /etc/grub.d/20_linux] at /etc/puppet/modules/xen/manifests/init.pp:27