Host VPS -- Netcup kvm : 4 cores and 16 GB RAM
dom0 NetBSD 10
domU multiple NetBSD and Linux for VPN, mail, web, Nextcloud, CalDAV, CardDAV

I'm a little bit unclear about memory allocation in a Xen domU. I can assign 1 GB or 2 GB to the dom0, and I would like the remainder available to all domUs, to balloon up and down dynamically as required.

If I specify mem = 2048 (for example), does that amount grow dynamically as needed, or do I need to add, for example, maxmem = 4096, and manually intervene to increase the domU memory allocation to this maximum amount? That would be tedious.

Or, by specifying mem = 2048, am I specifying a minimum amount of memory with which the domU is created, and leaving the domU to draw from the full pool of host memory as needed?