From the documentation on package naming convention:
Package names are written in all lower case to avoid conflict with the names of classes or interfaces
So this would leave you with the following two possibilities:
form_validator
formvalidator
Actually the documentation also makes it clear that underscore plays a special role when it appears in package names:
if the package name begins with a digit or other character that is illegal to use as the beginning of a Java name, or if the package name contains a reserved Java keyword, such as “int” … the suggested convention is to add an underscore.
So, underscore is suggested only in special cases, into which your naming problem does not seem to fall. So I would recommend formvalidator as the package name.