plpgsql trigger function check if all NEW.* fields exist in another table

In postgresql I need to code a trigger function that checks if all the fields in NEW.* exist in another table that has the exact same name as the table that fired the trigger but appended with _hv.

For example, if the table "my_example" fires the trigger I need to check if all the fields in "my_example" exist in "my_example_hv", and if it does not, alter "my_example_hv" to add the fields that are missing.

I am a very newbie plpgsql coder and I starting trying to code it with no success:

CREATE OR REPLACE FUNCTION trigger_hv()
  RETURNS trigger AS
$BODY$ 
DECLARE
    typeoffield character varying;      
BEGIN

FOR field IN NEW LOOP
      IF NEW.field not exists ON TG_TABLE_NAME||'_hv' THEN
        typeoffield := typeof(NEW.columns); -- of course does not work
        EXECUTE 'ALTER '||TG_RELNAME||'_hv ADD COLUMN '||NEW.column ||' '||typeoffield; 
      END IF;
END LOOP;

RETURN NEW;
END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;
ALTER FUNCTION trigger_hv() OWNER TO postgres;

I think I need some expert help...