Naming conventions of composed package names

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_validatorformvalidator

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.

The most conventional one would be the 3rd one: formvalidator.

The Google Java Style guide notes:

5.2.1 Package names

Package names are all lowercase, with consecutive words simply concatenated together (no underscores). For example, com.example.deepspace, not com.example.deepSpace or com.example.deep_space.

As @teppic said, the Oracle Java Docs state that

Package names are written in all lower case to avoid conflict with the names of classes or interfaces.

In Java package names are written in all lower case. Which means following would be the most ideal packaging name.

formvalidator

This is also accepted.

form_validator