Yes, I disliked KVM because of low-resolution problems, as I expected an auto-resize behavior as in VirtualBox, until I found out that you can adjust the setting in the VM, at least with XFCE, and it will survive reboot.
Very simple: Whisker Menu → display → change Resolution to 1920x1080
Then you can go full screen for a seamless desktop experience.
Works 100% on all my machines (desktop and laptop). Using it as I write.
The resolution is then hard-coded in this file (again, with XFCE, don’t know how it works with other DE):
.config/xfce4/xfconf/xfce-perchannel-xml/displays.xml
<?xml version="1.0" encoding="UTF-8"?>
<channel name="displays" version="1.0">
<property name="Default" type="empty">
<property name="Virtual-0" type="string" value="Virtual-0">
<property name="Active" type="bool" value="true"/>
<property name="Resolution" type="string" value="1920x1080"/>
<property name="RefreshRate" type="double" value="59.962844"/>
<property name="Rotation" type="int" value="0"/>
<property name="Reflection" type="string" value="0"/>
<property name="Primary" type="bool" value="true"/>
<property name="Position" type="empty">
<property name="X" type="int" value="0"/>
<property name="Y" type="int" value="0"/>
</property>
</property>
</property>
</channel>
See also: