User testUser = new User(stuff goes here);
testUser.UserPermissionsKnowledgeUser = true;
System.runAs (testUser) {
    //this works fine
    system.debug( [select id, KnowledgeArticleId from internal__kav where PublishStatus= 'Draft'  AND Language = 'en_US'  limit 1]);
}

global class testbatchimplements Database.Batchable<sObject>{
    global Database.QueryLocator start(Database.BatchableContext BC){
        return Database.getQueryLocator([select id, KnowledgeArticleId from internal__kav where PublishStatus= 'Draft'  AND Language = 'en_US']);
    }

    global void execute(Database.BatchableContext BC, List<sObject> scope){}

    global void finish(Database.BatchableContext BC){}
}

User testUser = new User(stuff goes here);
testUser.UserPermissionsKnowledgeUser = true;
System.runAs (testUser) {
    //this works fine
    Test.startTest();
    database.executeBatch(new testbatch());
    Test.stopTest();
}

global class testScheduled implements Schedulable {
    global void execute(SchedulableContext SC){
        Database.executeBatch(new testBatch());
    }
}
User testUser = new User(stuff goes here);
testUser.UserPermissionsKnowledgeUser = true;
System.runAs (testUser) {
    // fails
    Test.startTest();
    String cron = '0 0 * * * ?';
    System.schedule('test',cron, new testScheduled());
    Test.stopTest();
}