Hi,
The onHikashopCronTrigger trigger is a generic trigger which calls all the hikashop plugins each time the cron is fired so that the plugins can do what they want.
That means that if you implement this plugin, you'll have to handle everything in that function of your plugin :
the loading of the data you want in your KML file from the database with MySQL queries
the creation of the data to put in the KML file
the writing of the file on the hard drive of the web server
Then, each time the cron is fired, the plugin will regenerate the file.
Most of the plugins using this trigger have a mechanism in order to not process its stuff at each cron, but only when they need.
For example, if you look at the plugins/hikashop/rates plugin, which is used to update the currencies rates automatically, there is some code to handle that and an option to set the frequency of the processing. That way, even if the cron is triggered every two minutes, the retrieval of the rates is done only once a day.