I just figured out the way to install Xposed v88 on a Pixel XL phone.
If you have used Xposed on other diveces before, you probably know that the Xposed framework you are about to install is a .zip package , and you may want to use a 3rd-party recovery (like TWRP) to install it, just like you always did with other android phones.
However, the recovery method does not work with Pixel, at least at this time.
This is caused by Pixel's totally different partition plan, known as "A/B system partition"(which means there are two /system partitions). It seems that TWRP can't perfectly support Pixel's A/B partitions. (But why TWRP can install Super SU flawlessly into a Pixel? Anyone have an idea?)
An alternative way is using Flashfire to install .zip packages, and Flashfire supports Pixel's partitions.
So here's my attempt to install Xposed with Flashfire:
root the Pixel by fastboot boot twrp.img and install SuperSU.
reboot and install Flashfire.
copy xposed-v88-sdk25-arm64.zip to Pixel's internal memory. (xposed-v88 could not be the latest version when you are reading this)
run Flashfire, select the zip package, choose Slot A, check Auto-mount and Mount /system read/write
hit FLASH.
after reboot, install Xposed Installer (apk) and you will see Xposed is ready for use.
note: I reset my Pixel by flashing factory image (fash-all.bat) and I'm not using Magisk.