Preparation

In open Ubuntu 18.04 machine click Parallels Actions -> "Install Parallels Tools"
A "Parallels Tools" CD will popup on your Ubuntu desktop.
Open it by double mouse click, copy all the content to a new, empty directory on a desktop, name it for e.g. "parallels_fixed"
Open terminal, change directory to parallels_fixed (cd ~/Desktop/parallels_fixed)
Make command line installer executable (chmod +x install)
Change directory to "installer" (cd installer)
Make few other scripts executable: chmod +x installer.* *.sh prl_*

We have to modify some files:

Go to kmods directory (cd ~/Desktop/parallels_fixed/kmods) and extract the files (tar -xzf prl_mod.tar.gz)
Remove prl_mod.tar.gz file from that directory (rm prl_mod.tar.gz)

Changing file 1/3

Go to "prl_eth/pvmnet/" (cd ~/Desktop/parallels_fixed/kmods/prl_eth/pvmnet) directory
Open "pvmnet.c" file in a text editor
Edit line 438:

MODULE_LICENSE("Parallels");

MODULE_LICENSE("GPL");

Save

Changing file 2/3

Go to "prl_tg/Toolgate/Guest/Linux/prl_tg/" (cd ~/Desktop/parallels_fixed/kmods/prl_tg/Toolgate/Guest/Linux/prl_tg) directory
Open "prltg.c" file in a text editor
Edit line 1535:

MODULE_LICENSE("Parallels");

MODULE_LICENSE("GPL");

Save

Changing file 3/3

Go to "prl_fs_freeze/Snapshot/Guest/Linux/prl_freeze/" (cd ~/Desktop/parallels_fixed/kmods/prl_fs_freeze/Snapshot/Guest/Linux/prl_freeze)
Open "prl_fs_freeze.c" file in a text editor
in line 212 you will find a function "thaw_timer_fn":

void thaw_timer_fn(unsigned long data)
{
   struct work_struct *work = (struct work_struct *)data;
   
   schedule_work(work);
}

Change it to:

void thaw_timer_fn(unsigned long data)
{
   struct work_struct *work = (struct work_struct *)data;
   
   schedule_work(work);
}

void thaw_timer_fn_new_kernel(struct timer_list *data)
{
   struct work_struct *work = data->expires;
   
   schedule_work(work);
}

Then change a line 220 from:

DEFINE_TIMER(thaw_timer, thaw_timer_fn, 0, (unsigned long)&(thaw_work));

#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 15, 0)
DEFINE_TIMER(thaw_timer, thaw_timer_fn_new_kernel);
#else
DEFINE_TIMER(thaw_timer, thaw_timer_fn, 0, (unsigned long)&(thaw_work));
#endif

Zip it back

Go to kmods directory (cd ~/Desktop/parallels_fixed/kmods) and zip: tar -zcvf prl_mod.tar.gz . dkms.conf Makefile.kmods
Now you should see "prl_mod.tar.gz" file in this directory

Install!

Go one level up (cd ../ or cd ~/Desktop/parallels_fixed) and install it by running sudo ./install

In case of problems with Sharing Folder try to sudo chmod +x /usr/bin/prlfsmountd. (Suggested by @macdoor - Thanks!)

First two changes are hacks, changing licensing type and i cannot post complete, built package. Third file change was needed as backward incompatible changes were released in version 4.15 of the Linux kernel, on November 2017 by Linus. https://github.com/torvalds/linux/blob/master/include/linux/time.h

Kitty2014/parallels_tools_ubuntu_new_kernel_fix.md